Personally I would go for the 8 tube if you can afford it. Your post implied 7-10 plants, but not knowing what size you are going for it's hard to say for sure. I keep 6-8 vegging, usually switch to flower at around 2', but sometimes they get closer to 3' (I will continue to flower under HID, mix of MH and HPS)... As I mentioned, I'm about to replace a 250w MH I was using for veg with a 2'-8bulb t5 fixture, and I'm going to get a second 2'-8 bulb I can use on seedlings (the ones I'm getting have 4 tubes on each switch, alternating, so I can run half power for really early sprouts), and then move to veg, giving me 4' of 8 bulb. Besides more actual light (PAR preferably, or lumens, or whatever you are measuring in), the density of the bulbs will greatly influence penetration, and if you're trying to veg over 2' with T5's, you will be glad you went with 8 tubes. Another thing to keep in mind, 4' fixtures are going to give more light than 2 2' fixtures, I would only split them up if it's convenient for your growing conditions (like it kinda is in mine)...