Probably a better thread for the Hardware section, but here's my 2 cents. I have two sets of monitors (used to have three): Mackie HR824, Yamaha NS-10 STUDIOs, and Alesis (something model). I've gone the route of using them to A/B/C audio mixdowns, and I've come to the conclusion that it's better to
really get to know one set than it is to check multiple sets during mixing.
Even though one might attempt to be as objective as possible, and adjust critically to each monitor, it actually will confuse your brain. Sure, you'll hear different things from each one, but there's no point in trying to get a mix to sound terrific on both sets. If you learn the one pair really well, and begin getting some consistency with translating a mix to your car, a set of earbuds, and a big stereo system, you're going to find you get even more consistent with the one pair.
I sold my Alesis monitors years ago, and have the Yamahas in storage. I only use the Mackies. I'll do a full mixdown on those, and then I'll crank up my modified Polk 2.3TLs powered by a PASS X-350 to check in a leisure listening situation. If those check out ok, I'll burn a disc and go audition in my car and van. The car gives me the best verification regarding the kick drum, and the van will tell me if the kick is too punchy.
The bottom line is, I wouldn't buy another set just to compare with. Sure, you'll hear different things, but this won't always help you translate your mix to the world better. I would take that $400 and invest it into the room acoustics. Hunt down a local, but long-standing mixing engineer to your studio and have him/her give you some advice in your room. That alone is worth more money than you have budgeted, and all you'd need to do is offer a snack and maybe a drink. Pay for an hour or two of their time, if they need it.
With that said, many studios do have multiple sets, but the main reason is some engineers prefer one type/brand/model over another to mix with. Some even bring their own to work with. If you're going to shop for a set, I would save more money and look at upgrading to a set of ADAM's for example. There's some really good ones out there, but I wouldn't go buy another budget set. You won't gain much there.
Anyway, that's my take. Best of luck!!