Getting a small portable mixer for small things like weddings and such, where not just one band plays but lots of different performers switch, narrowed it down to Allen Heath CQ-20B and Soundcraft UI24R, since they're the only 2 that have built-in Feedback Suppressors.

Which one has better feedback suppressor ?

Have not used the Soundcraft but own a copy of the A&H and have to say the feedback supressor works quite well, I let it set a few permanent filters when ringing out mons and then switch to active filters with the shortest possible hold time... which is about 5sec total from set to fully released. That generally only allows a ring to just start to build before clamping down on it which is faster than a human can react, so the result is a system that sounds like it is very well under control.

First, I think that your criteria is a strange one for picking a digital mixer. Most seasoned mix engineers stay away from using anti-feedback devices because of their sonic quality effects.

Still, if that is what you need, then I wouldn't hesitate to recommend the CQ20b over the Soundcraft Ui24. Having used both (but admittedly I only putzed with the CQ and its feedback filters out of curiosity), and having followed the history of both, the CQ20b is likely a little better sounding mixer and even more likely to get more updates in the future. The Ui24 has only a single big failure mode I have heard about. It seems you can blow a channel under phantom power if you plug and unplug under power. Soundcraft and their new parent company don't seem to have any interest in this market so the Ui is unfortunately a dead end. It's too bad since it is a very good mixer with lots of reasons to pick it.

A&H have JUST released the CQ, have just updated the Qu and have a great track record for support.

I have a friend that has the CQ, I have both an X32 Rack and a Ui24 sitting next to me right now. Honestly, all 3 of them are pretty good units. It's just sad to imagine what a Ui24 mk2 might have been. Could have sold a bunch with just a few tweaks IMO. It was way ahead of the market when released.