The Vartag
is smaller than the .204, in fact
much smaller, and yet only slower by a measly 5%. The cost of powder isn't the concern, though for every 3 lbs of powder burnt shooting a Vartag you get one free so to speak compared to the .204 if you want to look at it that way. The noise/blast, barrel life, and barrel heat are far more reasonable (you can shoot more rounds in a string with a Vartag by good measure than you can with a .204 on a prarie dog hill before sitting out to cool the barrel).
My .221 Fireball, which is ever so slightly noisier than a Vartag and is the parent case for it, has much, much less muzzle blast than a .204 Ruger. Does it matter? Debatable, the .204's no sledgehammer, but is the smaller case nicer to shoot? Yes, hands down. The Vartag's even better.
Quite simply, the Vartag's a better gun, in my opinion, than the .204 because of efficiency and better case design. Is it better for you? Probably not if the cost of the Cooper or rebarreling with a match barrel is expensive to you. No off the shelf ammo either. For me, I like the best cartridge to do the job, the differences in cost aren't so much of a concern. I'm also a minimalist, I can't justify burning that much more barrel throat and powder to get a 5% increase, and I like high quality guns, so my Cooper M38 was a wise investment for me. Others mileage may vary...