I'm getting ready to install a new barrel on my flattop receiver. Is it better to use a receiver clamp or barrel jaw blocks?

In my opinion, it is WAY better to have the upper reciever vise block set.  The clam shell portion holds the reciever and the insert keeps everything intact.  With this set-up you don't have to worry about deforming the barrel pin notch in the reciever (which is very commonplace with the barrel vise inserts alone).

You will probably want a set of the barrel vise inserts though, for holding the barrel to change and/or remove the front sight base.  It makes that job much easier having the barrel solidly supported.

Totally agree, hold the reciever. That is what the barrel nut is screwed on to. Good answer!!

Given the amount of force we needed to get the barrel nut loose from my Arlalite upper even WITH a receiver block, I can't imagine doing it with the barrel jaw blocks.  I had to sit on the workbench, while my buddy, who goes about 300lbs and works as a bouncer pulled for all he was worth on the wrench.