I just bought a SPS Tac in .223, it will be excusivly for Ground dogs (& gophers) and coyotes. It is a tac driver (imho), Cheap FMJ ammo groups at an inch at 100y, I'll have to let you know how Noslers are after I work up a load.
I already own remmy 700's in 30-06, 270 and .243 that are more 'suitable' for big game hunting so it really wans't an issue for me.
If I was in your shoes, I'd buy the 308.... Especially if you are planning on hunting larger game (deer sized or bigger)
I am planning on getting a 308 in '10, after christmas, I was thinking about another SPS tac or a VTR (I just love the look of them triangle barrels, and the 700 actions) and I am already set up for 30 cal (bullets and primers) all I really need is brass..... So that will make it a bit easier for me.
As far as it being 'cheaper' to reload for .223 as opposed to 308, it defiantly is! Compare 250 50gr Nosler BT @ $59.90 compared to 100 185gr Berger VLD's @ $79.96. plus less powder, primers are primers and there is very little cost differance there. Just bear in mind that you can still reload 'quality' ammo in 308 for less than factory 'quality' .223 ammo. So what I am saying is, it's cheaper, but not by "that" much.
Have fun with your decision, I would really consider buying both......
Cheers!