When I bought my first Tic Tac in 308, the varmint was nowhere to be found. It was also a question of what components I could find. The Tic Tac is a great rifle, and I would say it is as accurate as the best factory rife from any manufacturer.
Given that the workings and the barrel are essentially the same, (minus the threads for a brake and the parkerizing) I strongly second the vote for a varmint in 6.5X55. It is a superb cartridge and the tikka heavy barrel rifles are capable of great things. (My 308's consistently shot .3 groups at 100 with hand loads) But to milk the most out of it, I had to seat on the lands and that made the rounds far too long for the magazines.
Ballistically, the 6.5 bullets are generally superior, and the 6.5X55 is a superb cartridge out to 1000 meters. Given the light weight of the gun, it is kinder to shoot all day. You are not as likely to find a box of 6.5 factory ammo at "bubba's bass and bullet store", but then if you're shooting factory loads, don't expect accuracy to be as good.
As good as they are, you will basically have to content yourself with the rifle as she sits. Aftermarket stocks do not exist, and the weird recoil lug configuration (integrated in the stock, not the action/barrel) makes finding an alternate very difficult if not impossible without lots of modification. This also impacts bedding the action. The rail is nice, but it is not tapered.
Thoughts to ponder.