About 3 or 4 years ago I bought 2 Novatron M500 kits (a total of 4 lights). I don't remember where I first heard of the brand, but somebody, somewhere in my past used them. It was the only brand I was familiar with so I bought that brand.

Well earlier this year, I attended a workshop and the instructor was using Alien Bees. After reading a ton of stuff on this site about Alien Bees, their price, and then seeing them in action, I was sold.

Now I have recently rented studio space again and am in need of 2 more lights. Buying another Novatron kit will cost me around $1200. If I decided to buy Alien Bees, it would probably run me about half of that for the same set up.

The thing is that all of my barn doors, snoots, gel holders, speedring adaptors, grids, etc, etc all fit my Novatron lights.

My sense tells me that since I already have Novatron lights, that I should just ride it out and buy another kit, but my desire is to have the Alien Bees.

What do you think I should do? What would you do?

I'd look at the system as a whole rather than just the lights.

There's nothing that says you cant run both. I have 1 AB400 I use as a hairlight, 2 whitelightning Ultra 1200's I use as Main and fill lights and an old Novatron 440plus pack with 3 heads that I use as backround and accent lights.

I don't know where the $1200 number is coming from, Adorama ans B&H have M500's for $459.  This is reall an all or nothing thing.  You want one system with interchangeable everything.  The Nova's have 6.5" reflectors, the AB's 7", nothing will interchange and you'll drive yourself nuts keeping two systems straight.  You want to be thinking about the picture and interacting with the subject, not tide up in musical modifiers:

"Hmmm, I need to put a light back there for rim, but crap! it needs a snoot and I need the other brand for that, so I can swap out the main but I wouldn't have the right speedring, maybe the fill then, but will that light dial down far enough? ... Hold on a sec while I change almost everything!"

My suggestion would be to stop, take a breath, and not make any decision until you do more research.  You tried one brand and had been happy with it, now you've tried another and are thinking of changing.  I'd say try more and/or read more.  It's like an old friend had a CJ5 so you went out and bought a Jeep Cherokee.  Now you've ridden in a Kia Spectra and realize that you can get from point A to B for less.  Go try a Toyota, VW, Chevy, Dodge, Saturn, etc.  Or at least look over their price and spec sheet.  There are a bunch of great strobe brands out there Profoto, Elinchrom, Hensel, Bowens, Speedotron, Dynalite, Blacar, Broncolor, to name a few.  Which manufacturer has the system that best fits your needs regardless of price?  It's an important question because you can waste more money trying to fit a less expensive square peg in a round hole than if you just bought the round peg in the first place.

I picked up a Novatron 550 kit w/3 lights off ebay for around $300. So I would check there. As you know they're not the best nor the most durable lights. Although at the end of the day they do the job you need them to do. I originally wanted a set of Alien Bees, but couldn't beat the price of a used set of Novas.