The fist rule of digital multi-FX is that the price of last year's model drops considerably when a new model comes out.
1. If you're running direct to the PA, presumably you want amp emulation as well as FX. Every company swears every model is just like the real thing, but they vary considerably and not consistently from amp to amp and company to company. So you want to make sure the type of amp emulation you actually want sounds good enough to you. Same with FX.
2. If you're running to the PA, you want separate outputs for your amp (if that's what you're using for monitoring) and a direct send for the PA. The amp out should be appropriate for a clean guitar amp/speaker and the PA out will be add more stuff to make it sound more like a guitar amp. Most of the serious entrants have this functionality.
3. Ease of use and editing are important.