well, Josh Perkins,
I’m going to buck the trend…
not that anything said here so far is wrong, or incorrect, or distasteful, etc…
But I would have to say the bottom reg would be my choice If I were in your shoes, ( I went from practice set to full set, BTW)
and here’s my reasons:
1.) It increases the range of your chanter downward. (c,b,a,g) Musically this makes sense, as there are no notes on the other 2 regs that arent there already on your chanter.
2.) You’ll be able to use your right thumb (B+C), right pinkie (G &A)
as well as your wrist to get notes. Not that one cant do this on either of the other regs, but with just the bass it’s an easier ‘target’ until you get fluent on the things. (& I hope you will be able to get all three one day!)
3.) With forethought, one can imply much of the same harmonies with just the bass reg… OK, you wont be able to do that wonderful g-f# over a d (tonic sus4, in the biz), INSTEAD you will be able to get that c-b-suspension in the key of G but all the most common chords are present, Am, Em, Bm, GMaj, DMaj (with a in the bass: tonic 6/4 in the biz)
3a.)Ive also found it fun to do moving contrapuntal lines in contrary motion on the bass reg: (b,a,g on the chanter against G,A,B, on the bass reg) Leo Rowsome did a lot of things (zampogna-like things, actually) like that on several recording s Ive heard.
4.) and (THIS IS JUST MY PERSONAL TASTE:) those woody low tones combine wonderfully with the chanter’s bottom register, 
My bass reg has probably needed the least adjustment over the years; & unless I’m doing something like playing a sustained chanter hi b, the bass reg for me, is least likely to utter a discouraging tone…
So, one vote for the bass reg, take it or leave it. 