Having 2 handles with the rubber top is nice for lowering them down in tight spaces, but I'm sure I could manage with a single strap as well. I don't stack kegs but on the "flip side" the rubber tops are nice for draining the kegs if necessary, you can just flip them upside down and walk away and the keg balances itself, can't do that with the single strap. Not sure if that matters to many or not.
The worst part about the rubber tops is when they start to break down and get black smears on whatever it touches. However, I'm guessing that's not really an issue with brand new kegs. Those things are straight from the factory and going to be inside and babied by us homebrewers their entire lives, quite a difference from the used kegs that are 30-40-50 years old and have been through whio-knows-what-and-where before they make their way into our homes.
In the end, I don't have much of a preference either way for the 5's, probably would go for the single strap to save a few bucks and see how I liked it overall before deciding to buy future ones with single metal or double rubber handles.
As far as 2.5 vs 3 gallons, I would probably opt for 3 simply because the price and size are almost the same, except you get an extra half gallon capacity. As someone earlier said you can get 2.5 into a 3, but not vice versa. And when you do a "half batch" you don't always end up with 2.5 gallons, sometimes it's a bit more and we hate to waste any available drops of beer. That extra half gallon gives you some leeway without really being much bigger in overall size.