Definitely don't buy a regulator based on how it looks, unless of course you intend to use it as a mantlepiece.
Regarding the size of the G250, the mouthpiece is the same for all SP regs, and if you don't like any mouthpiece on any reg you can easily replace it. I like the seacure orthodontic mouthpieces. As far as the size of the reg itself, remember that in the water, the 2nd stage is filled with air and so the slightly larger size does not mean extra weight or tug on your jaw. Salesmen love to sell small regs based on "jaw fatigue" but that's basically B.S. Hose length, routing, and mouthpiece design are the real factors in jaw fatigue. Using a long hose helps quite a bit.
Of the three you mentioned, I am really familiar with SP regs and so I tend to recommend those, but of course that's only my experience. I would, however, avoid the SR1 like the plague unless it's dirt cheap. I have a friend who bought one and it creeps 10 PSI right out of the box. If I spent $500 on a reg and it did that I'd be pissed off. Maybe his reg will lock up better after a few dives, but if they haven't gotten the seat material and/or piston design right yet, why bother? A well serviced MK25 will lock up with a VERY stable IP and not budge for years of normal use if maintained well.