Thanks for all the replies!
I just came back from a trip to Epcot and realize that a flash is not really feasible for travel.
I would not be able to put the camera in and out of my bag without having to take the flash on and off all the time. This would make it very slow and difficult and I know I would not even bother with the flash after a while!
Also I was looking around at other people with SLR's and not one of them had a flash (except the Disney photographers, which are stationary)
I also realize that the 15-85 range is amazing for travel!! I brought my 55-250 and only put it on once, for a couple of shots, just because I had it.. but really could of lived without it. Good thing its very light, but next time I would rather bring a more useful lens unless I know I am going to be shooting telephoto.
So I realize I really need some low light capability. At night and on the rides it was a challenge with the 15-85. Most of the time you could not use a flash and I was at 3200 ISO (I even went to 6400 a couple of times)
Looking back most of my indoor shots were a lot of different focal ranges (many at the wide end) and I doubt the Sigma 30mm would of been usable since you do not always have a choice to move around indoors.
My question is, if I had the 17-50 Tamron f2.8 nonVC for indoor use how much would it of improved my ISO or pictures?
Here are some examples:
1. Some of the really dark pictures from the rides I was at f3.5 at 3200 ISO and they were still not great. What the f2.8 of been able to capture the shot?
2. If I had indoor pictures at 3200 ISO from f4 to f5 then with the f2.8 what ISO would I get and would I be able to avoid using a flash?
f/2.8 would not be sufficient for what you want to do. f/1.4 might, but then you'll be focusing on 1 person wide open for rides, which is usually good enough. f/4 is one stop from f/2.8, so you would be at ISO 1600 at f/2.8 instead of ISO 3200. f/1.4 would get you down to ISO 400 with a thinner DOF.
Fast primes give you larger apertures but you don't get focal length flexibility. I think you should look at the Sigma 30 f/1.4. It'll work better in low light situations that you found at EPCOT. The flash would still be a better option for wedding situations, where the rooms are dim and you're often taking group shots. Ideally, you would like to get both a flash and the Sigma 30 to cover your bases.