Not buying a left back because he has to "pay on top" for Messi makes no sense. It really doesn't.
I can see your angle, of course, as far as stopping him completing his side is concerned. That's not really a money issue though (unless the entire point is "you can only get Messi if you keep a shit LB").
One being better than the other is nonsense at this level. I've been saying it for days now, at this stage "better" is defined by fit into a system, not individual quality, records or awards. Both players are worth as much as each other to either team, quite clearly. Without them, neither looks complete and with them there's no room for improvement. That's not really a money issue either.
So it boils down to you would only let him have Messi if he keeps Capdevila at LB. Of course I wouldn't agree with that.