One of the best things about a decent blast gun is a good selection of quality nozzles and jets to accomodate different media types and/or air supplies. Otherwise, ergonomics, flow, etc. are considerations.
The biggest thing to consider is whether you want a pedal operated gun or one with a trigger. For long blasting sessions, a pedal is a great feature. But it requires additional hoses, fittings, and of course the pedal.
I think the TP tools guns are great. Lots of tip options to choose from, well made, and available in trigger or pedal configurations.
Trigger & Foot-Pedal Siphon Guns & Parts for Abrasive Blasting Cabinets
www.tptools.com
All that said, the gun is only part of the equation. If you are experiencing clogging consistently, you might have too much moisture in your media. That's a problem you have to address up stream with your compressor, filter, drier, distribution system, etc.
As for a metering valve vs a pickup tube, yeah, you can improve your performance by using a metering valve, mostly because you can fine tune the mix of grit and air. I don't see why you can't still clear the gun by covering the tip. It just takes a second and your not going to blow a bunch of grit out of the air inlet.