I'd choose the 138, it is because I would love to PVM get the new firecape achieve anything and everything, that is so much easier when your maxed combat.
Honestly its a hard one. I suppose there IS pros and cons. a pro for the 138 is that you can easily make money by bossing and doing such and such, but a con could be that it can get recovered.. or if you get cleaned its really hard to rebuild. with the 1b the pro is that you can merch to make more as well.. but you'd have to spend it all raising an account to the lvl 138 status. i think i'd choose the 138 imo.