First, I have had really good experiences with AI Sols and AI Vega fixtures. I am a big fan of them, even after looking at the other LED options at MACNA, and I am going to stick with them. I like the modularity, clean look, customer service, and for the Vega, the 20 channels of control and upcoming custom puck options and AI Director interface. That said, I have also mounted mine pretty high AWL and that may not be an option if you have a hood and are trying to keep the lights in the hood. Closer to the water line would call for more fixtures.
Assuming standard 175g bowfront dimensions of 72 x 24 x 29 and a good mounting height of say 10" AWL I would think a total of 4 Sol or Vega fixtures would work. I would run three of them lengthwise and add a fourth to the center of the bow area if needed. If you have a deeper tank, which it sounds like you do, the Sols may work better for you as they have the 40 degree optics in the center and 70 degree optics on the sides which can be swapped out for 40 degree optics. The 40 degree optics will give you a lot of penetration and the generally higher PAR values will also help with depth if you plan on keeping light loving corals toward the bottom. In comparison, the Vega offers 80 degree optics but you can probably get them a little higher in a hood because they are not as bulky as the Sols.
You can get the Sols for less, about $400 each new and I've seen them around $300 ea used. I have not seen used Vega fixtures yet so you're pretty much stuck at $500 a pop. You'll also need to add-in another $80 for a controller. If you start with 3 of each, then it would be either $1280 or $1580 for new lights.
As you've noted, bulb changes are not cheap and you'll get some electricity savings from running fans or a chiller less in addition to the savings on bulb changes to help defray the initial purchase price. If you were to change bulbs every 6 months, you'd pay for the lights in bulb changes alone in about 3 years.
Let's say you go with the Vega fixtures for a year. Even if you don't like them and go back to MH, another lighting system, or even DIY LED, a couple of bulb changes alone would make up for the difference in what you could sell used fixtures for. So, in my mind, getting off-the-shelf fixtures is a no brainer if you have the initial funds to do it and give it a try.
What you may want to do is try out some of the fixtures or see how they do on other people's systems before committing completely one way or another. I'll be offering up a Vega fixture and controller for loan to club members who leave an appropriate deposit so that they can try them out over their own tanks in about a week or so.
