I think Moviebuf has it about right.
The Brenkerts were great machines... probably the first ones I ever ran. There are a lot of them still out there, running fine, but getting reliable parts is a problem. You could say the same for a Motiograph... I know of a few of them, too.
Personally, I think Century/Simplex is a toss-up. I have Simplex here (XL & 35 models), because I have a dusty environment & I figured a sealed machine would be best. They all do leak though... if you can handle that, they're rock-solid battleships.
Century machines are probably interchangeable with Simplex, quality-wise. The "C" models are a little old for my liking. I'd go with SA or DA models (single or double shutter). You can also get them with curved gates. Also, the sprocket shafts are supported by bearings, not bushings, like the older 'C's. The trade-off is that the Century gear train is not sealed. You just open a door and it's all there. I used Centurys in my indoor, but was concerned about keeping the grease clean in my drive-in. Because of that, I went with Simplex, and the associated cat litter trays under them! I might have been a little more concerned with the thinner Century shutters at the drive-in (heat-wise), but never put them in service here to find out. I've seen enough warped ones though, so you might have to be a little more careful, putting one in front of a 4-7,000 watt lamp.
Some feel that Century parts (bearings, gears, etc) don't have the service life that an XL does. On the other hand, being open, parts replacement is easier and faster than pulling an XL apart. In my experience, Century projectors don't wear so fast that this was ever an issue for me.
As for the associated sound heads: I use older Simplex 1012s. They've been here for 22 years, were old then, and do the job just fine. Some prefer Century, because unlike a Simplex or RCA, there is no roller, pressing on the film.
In the end, I don't know that there's enough of a difference between brands to get excited about. You'd probably be as happy with one as the other.
[This message has been edited by rodeojack (edited July 11, 2008).]