If Mesa isn't hiring, there's no sense in doing the PACE program, since the only thing of value it buys you is the interview at the end. Other airlines won't touch you with only 300 TT unless you've done an internship with them. MAPD/PACE allows you to skip the CFI/freight hauling phase most people do to build sufficient time for an airline job.
What's in it for Mesa, you ask? For starters, it minimizes wasted training dollars on people who wash out of training since MAPD/PACE'rs have already been through the CRJ systems course Mesa's newhires go through, and thus aren't as likely to bomb out there since they've been exposed to (and tested on) all the material before. It also maximizes MAG's training dollars in that a low-time pilot will be in the right seat longer than a higher-time pilot who's already met the mins for their ATP cert (1500 TT, 500 XC, IIRC) and might upgrade to the left seat sooner.
Presuming Mesa's hiring pace keeps up, PACE is a no-brainer, particularly for us older peeps staring at a flying career half as long as the 20 yr old Riddle grads. If US Airways tanks and a huge chunk of Mesa's business with it, all bets are off.