If you can get away with the larger cooler and maintain a decent grainbed depth (really only important if you're using a manifold anyway) then you should go with the 5 gal cooler. When, not if, you decide to transition to all grain the 5 gal cooler will work for virtually all your 5 gal AG batches. Then, when you step up to larger batches you can use the 5 gal round cooler for a sparge tank.
But hey, I'm just planning ahead here. Like you, cost is a factor for me, but you can get a cooler for very cheap at walmart. Yes I hate them but I'll bite the bullet and shop there if it'll help my brewing.