This depends on what you need.
LimeWire is the more "advanced" client, but it is being created by a company which has its own goals, which sometimes get into the way of the best possible software for their users. The fulltime developers do give it a boost, though.
It does consume quite a bit more memory and is harder on your MacBooks processor (at least it was, when I checked one year ago).
Phex instead is a lightwelght program which offers more control over its inner workings, and it is developed completely in the open by a group of developers who work on it in their free time.
Do you want "I just click and hope it works" (LimeWire), or "I can choose how it works, but it doesn't look as simple" (Phex)?
What I especially like about Phex are download strategies, where you can decide which parts of the file Phex downloads first - and anonymous p2p: Phex is in the process of being integrated with the i2p decentral anonymizing system, so for the long haul you might be on the safe side with Phex (you'll have to find i2phex yourself - it isn't ready for users who are reluctant to search for it themselves, yet).
We created a comparision of the features of several clients, some time ago:
http://www.phex.org/wiki/index.php/Features
But in the end, you have to decide for yourself.