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The 48 is bigger. Fairly "typical" PT rim. And it's bigger. If you are wanting to play in PT land, I suggest you work your way up to the 48 via: PT44 or 42, then PT50. The if PT50 is still too small (which it won't be), then go for the PT48. One other thing about the PT48 is that it's bigger.

Bob, the PT48 is the mouthpiece I used in my army band days on a 5-valve 187. It worked really well. It's a big one, no doubt, but produces fine results on that particular horn, at least for me. I asked Mr. Tucci to make one for me with a slightly sharper inner rim, and he kindly obliged.
The 18 is a very nice mouthpiece, as well, but I have no experience with the 12. I'd just play the 48 exclusively for a while on the 187, it should work. Bouncing back and forth won't help too much because it will increase the time it requires you to "settle in" on the mouthpiece/horn combination.
FWIW - I also used this horn and mouthpiece in the Brass Quintet, with no ill effects. It depends on how the quintet plays and how good the tuba player is . .
Did you find a 48 yet? If not, PM me.