to clarify...
if you've ridden competitive DH and race BMX, we can assume that you have a modicum of bike handling skills. This is why I suggest 26in wheels would suit you better. Try to ride a 29in bike, both on a smooth-ish, flowy trail (where they are great), and on a techy trail with off-camber corners, step-ups and tight, uphill switchbacks (where 29ers are friggen horrible).
I rode 29in bikes for 3 years, mostly on the east coast. I had three. Only one of these handled nicely. All were popular bikes. Canberra is flowy (read: easy, some might say "boring" or "gumby friendly") and 29ers are really well suited, which is why everyone has them. Same goes for some, but not all east coast endurance race courses. It's easy riding, it's open, flowy trail. 29ers can be quite nice.
I moved to Adelaide, the trails are much narrower and not manicured at all, the climbs are much more pinchy, there are a bunch of off-camber corners. 29ers such donkey dick here, which is why nobody has them.
My thoughts are that if you ride a variety of terrain, and are a solid technical rider, 26in wheels are simply more versatile. They might be a fraction slower in *some* situations, such open, flowy descending (where there is very little time to be made up in a race situation anyway) but they are never downright horrible. The same cannot be said, in my experience, for 29in wheels.
xc duallies are generally moving in the direction of 120mm travel, rather than 100mm. Bikes do this about every 5 years. 5 years ago, most xc bikes were 80mm. Before that, a lot were 63mm. Suspension technology is getting a lot better. You can very easily get away with a 26in wheeled, 5in travel bike for xc racing (unless you're at or above the pointy end of a-grade, but most of those guys are on hardtails anyway). This is good news for riders like you. 26in wheels, solid pedalling platform, more travel.
It is horses for courses. Get the bike that you like. If you can, try to have atleast a car park test. Not all bike shops (not even most) can just lend out any bike in your size for you to shred on for the weekend. If you've ridden a bunch, you should have a pretty good sense of the feel of a bike. Sometimes a bike just has a really nice line and length to it and just feels really good under it. Get the bike that works for you, wheel size is a very small part of what makes a bike good or bad.
Also, fwiw, asking these kinds of questions in 29er sub-forums will very rarely get you good advice. Mostly it'll get you something along the lines of "get the one with the big wheels, becuase they're better *insert joke about training wheels here* blahblahblah, now lets go make man love after we spend 5 hours looking at pictures of 29ers on the internet..."
