Hi Peter
I have played all kinds of different cab combinations 10s +18s (bi amped) 10s + 10s

, 10s + 15s and
15s +15s and even 12s and 18s. (and I confess that I even used different brands)
I was always on the run from the mixed cab/ speaker police.
Every combination can sound good. But like Andy said:
It's always best to try before you buy! It can be expensive if you don't. (ask me how Iknow

)
You can't go wrong with something you like! => start here!
You get one more! More is always better!

(see below)
Even with two identical cabs you get two "tones" or rather sounds
One sound from the cab that couples with the floor and one different sound from the one on top.
It's the room that does this. Even with the same volume this combination will sound bigger than one cab.
Because one cab can't be in two different places at the same time. (makes that sense?)
You can and should try different placements for your cab! Lifting it above one foot can lighten up your sound
Decoupling it from the floor will make your sound less bassy.
You also might hear yourself better because the speaker is closer to ear level.
Adults use a beer crate

(or two next to each other)
I think if you get two 115s (that you like!) your Mesa will sound great!
I also think you should go with two 8 ohm cabs (= 4 ohm together),
and try them with your PF50T. That little beast sounds awesome with two cabs!!!!
have fun
Wise(b)ass