Ruined: You are a smart person. I like smart people. Thanks for coming to the forum I am going to go out and do a photoshoot at 250 and see if I can tolerate it... This might be a diccy (& fight provoking) question, but, how much could you 'crop' a full frame image to say it is the 'same' quality as a crop. Rather than 250, could it maybe closer to 300/320? oh god my brain hurts and I'm not even trying to work out the answer. I guess there are probably many variables...
There have been quite a few tests done on this, even here at CR.
To sum it up, if you have good light (i.e. ISO 1600, maybe 3200 if the 7DII improves over the 7D - which initial tests seem to indicate) and you are reach limited (meaning that in this case, you are taking a picture that does not fill up the whole frame of a crop camera when using your longest lens), then generally crop will win. If ISO is higher (3200-6400+) full frame will probably win with cropping, but do remember if you are at 300+mm with your 100-400 frequently you probably want to stick with the crop. Otherwise, its going to be a real pain in post to crop every shot, and cropping aside the subject may be too small in the viewfinder to take the type of photos you want; I see a lot of emotion in your wildlife photos, that may going to be hard to capture if it is too small to see due to lack of reach when taking the photo! If you have Lightroom (and some other programs), you can set a filter to see what focal length your entire library of photos is at; see what percentage of your photos are greater than 250mm focal length to get an idea how many you'd need to crop on full frame to get the same picture. You can do the same with ISO to see how many you need ISO higher than 1600-3200.
Full frame is quite excellent, but crop has its applications too. That is why they make a pro crop camera, and the 7D line is known to be popular with wildlife photographers due to the extra reach. Full frame has an allure too, for my portrait work I would never give up full frame. But full frame is not a panacea that conquers all, it is simply another option that mostly offers significant improvements, but does have some notable drawbacks as well for the type of photography you shoot based on your website photos - reach being the largest.
IMO in your case there really is no definitive answer. If you want only one body, you will have to pick between reach and high ISO! So, you should check the usual focal lengths you shoot too see what percentage >250mm, what percentage >3200 ISO - then do some shoots capped at 250mm on your current camera to see if the reach is sufficient. If photo analysis and your tests indicate 250mm on your current camera will be enough for 95%+ of your photos, then definitely go for the 5D3. If you exceed 250mm frequently and need the reach, I would think twice about buying a full frame camera due to all the cropping that will be needed - plus a good deal of the benefit of full frame (not all) is lost when you crop lots of the frame away every picture.
In short, do your research on your own photos/photography to see which solution is best for you. Don't buy full frame just to buy full frame, as you may be disappointed if you want/need the reach of crop.