Combo's are always open backed, with a stack you have the option of going closed back with the cab. A closed back cab is usually more focused, tighter. Also with a stack you have to option of playing with different head / cab combinations. Also in favour of a stack is that you can put an atenuator between the head and the cab, this is not relevant for low wattage amps (I love the TT by the way), but if you want a 50 or 100watt amp a combo is no good for practice. I had a Peavey 50w ValveKing, nice amp, but I couldn't work the valves at all in a flat.
So I'd say stack, hoever stack is more expensive, and the tempation to try new things is a terrible drain on your wallet!
gwEm makes a very good point, single speakers are all very well, but more is better! I used my TT through a 4x12 and it did really well, better than through the 1x12 I normally played it with. Both cabs were Orange, and both had V30's in them.