What system do you think you'll have in the future ?
I ask as it's about the same price to get an AI fixture as it is for a high end T5 ( 6 bulb or more ), and will cost you less in the long-run as there won't be any bulbs to buy
If you want LEDs on your future system, consider getting a single now
Otherwise, go with the 2xT5 suggestion, although 4 will give you more bulb options and won't limit your needs