It depends on the situation.
First of all I wanted to talk about bags, I'm a self confessed bag-a-holic if you have a good bag weight is a lot easier to deal with. Definitely worth looking at your bag set up before you ditch your gear for smaller and lighter variations. I generally spend a lot on bags, £250-400 is about where the best bags are. There are many under this price but they are usually a lot heavier. A general bag can weigh 2-3kgs without any of your gear in it! Thats basically 2x 5DMKIII and 24-105mm combos. So making sure you do your research and find the right bag is paramount.
My favourite new bag is the Fstopper Loka UL (ultralight) its a 37l which is a modular meaning you can switch out the internals to your situation. There is a small med and large internal camera storage options or you can not bother and use the bag on its own. The bag weighs 1.2kg with a large ICU inside. The UL does have a few drawbacks that the inside compartment doesn't have a partition so things you put in the top can fall through and it doesn't have many zipped sections. But its a big bag which will take 2DSLRs 4-5 lenses flash up to 15" laptop and enough gear for you to be on the mountain for the night.
As I've grown as a photographer over the last 10 years its more about what I'm shooting that decides my gear choice.
Professional:
When it comes to my professional work weight is an issue because when you are shooting a wedding for 8-12hrs 8-12kgs of gear does take a toll but I also take the best gear for the situation.
Weddings and events: I always take two Full Frame bodies (5DMKIII) one with a standard zoom at F2.8 (24-70mm) and one with 70-200mm, then I have 90% of situations covered. I then also carry a 16-35mm F2.8 for those situations where space is cramped and still low light. I generally like F2.8 lenses for this as it gives me a specific look with the versatility of the zooms covering a large focal length. I also take along a prime with me usually a 50mm just for portraits and a twin flash set up.
Motorsport: Same as above minus the 50mm + 100-400mm. Motorsport events usually are over weekends, so there is the track action and then the enthusiast cars usually on display.
I take one Full Frame body and one Crop (5DMKIII and 7DMKII) The crop body because they offer faster shooting speeds and are smaller bodies which I prefer. When walking through the club fields I will carry the 5 with a 24-70mm or the 16-35mm and a 70-200mm on the 7. When on the track I switch the 7D to the 100-400mm and stick the 16-35mm on the 5. F2.8 gives lovely images of cars and also reduces fussy backgrounds, while on track its not as much of an issue when panning.
Personal:
For my personal work weight is much more of an issue.
Landscape: Although I do shoot landscapes for clients its not my bread and butter. Generally take one FF body, 16-35mm F2.8 24-105mm F4 and I used to take my 70-300mm L before I sold it. This gives me a great range of shots that covers most aspects from wide to more tele type landscape imagery, they are slower but lighter. The 70-300mm is a great choice as its small offers amazing quality and can sit upright in most bags. For landscape work I generally stop down so having fast lenses isn't necessary except when shooting at night which is why I keep the 16-35mm as 2.8 is very useful. This is to keep weight down when I'm hiking I like to keep the weight down and also the amount of gear because I will take supplies depending on the day, could camp or just take some food etc sometimes I take a tripod depends.
Travel: I traveled North and south america for 5 months this year and I definitely wanted to travel light. I took the 5 with 24-105mm because its a great all round lens, 16-35mm F2.8 which is also very useful can be used as a nice 35mm when needed at 2.8 and can be used for landscape and nightscape, 70-300mm L I went to the amazon and quite a few other areas where there is abundant incredible wildlife so I wanted to take a very sharp zoom which is also very compact, loved it apart from it was a little short. I also took an iPad mini, 2 back up hdds on 2tb and a 1tb hyperdrive you can put images straight onto with the built in card reader. This all fit into a small 22l day bag and I carried another large 60l bag for all my other things.
This year I'm going to africa for 2 months and south east asia for another 2. This time I'm taking more, taking the 5D and 7D 100-400mm instead of the 70-300mm and a 1.4x extender. The 70-300mm was just too short for wildlife and I want to take two cameras with me so I can have a wide angle perspective and a more intimate perspective. This gives me 16-896mm which will be fantastic. Im also replacing the iPad with my 11" macbook air because the colour from the iPad mini was atrocious, it was fantastic apart from that. But this is much heavier about 4-5kgs more but at the same time I have two bags so I can move items from one to the other when I don't need them.
So basically if i don't need to take heavy F2.8 lenses I won't. I have a large range of gear which covers most situations and sometimes multiples of the same focal length but lighter and smaller versions 24-105 and 24-70mm 70-200mm 70-300mm and 100-400mm. Most people don't like to have multiples of the same focal lengths but if you use the gear I don't see why not. I shoot professionally so I do have an excuse to buy more and not worry about it sitting around, the cost also does not bother me as each item will pay for itself many times, if not in money then in enjoyment. There is a downside - if I'm using one the other is usually in the cupboard. From my lightroom catalogue I have found I use most of my lenses about the same because of the various types of photography I produce.
In terms of bodies, I have medium to large hands and anything from 70D up is the right size for me. They are comfortable, ergonomic and i enjoy using them
I find the mirrorless offerings don't play well with the focal lengths I use (which is most). I would love to use the sony gear but putting any L lens on them with an adapter just doesn't work the same way, the ergonomics are no where near as nice as a proper canon body. The mirroless equivalent lenses don't exist either, the 24-70mm Zeiss is by all accounts a poor lens, the 70-200mm is only available as an F4 and there is nothing above 200mm adapting lenses just isn't my idea of reliance, I need to rely on the gear and this just doesn't give me the same performance.
The battery life is also not good enough for me I carry 4 canon batteries all the time with me and those sony batteries last about 1/4 the time. The new mirrorless bodies are also not that much smaller now as they grow with the needs of the people that use them. The positives don't outweigh the negatives for me yet. Plus I just love my canon gear! The new A7RMKII isn't much smaller and lighter than a 5DMKIII.
I have also come to rely on my gear I go to hostile places, out in the rain etc and the canon gear is solid and I trust it.
