I've owned EVGA (currently a 7900GT oc'd to 550/1550) and BFG (a 6800GT Ultra AGP and a 7800GS AGP) products and both are top notch. Regardless of which on you choose you can be assured of a quality product that performs well if not above expectations.
However, out of the two, I'd go with EVGA. The Step-Up program is an easy and inexpensive upgrade path but most importantly, EVGA's warranty covers the use of after market coolers as well as any failures as the result of overclocking. Chances are, my next gpu, when I make the move to a DX10 product, will be an EVGA.
I haven't purchased EVGA, I can only speak for my great experience with BFG. 24 hour tech support meant I could (and did) rma the 6800gt I had at 3am on a wednesday, ship it on thursday, and get a 7800gt back a week later.
I will certainly buy BFG again.
As for EVGA...
I don't think you can go wrong with either company really. EVGA was also the better company to deal with of the ones who had overheating 7900s a while back.
Their 7900gtos... if you could get them, were a great idea and an amazing price for it's time. Something I wish more companies would do, be a bit more risky with the reference designs beyond just overclocking. Pairing the 7900gtx core with the (512mb no less) 7900gt memory made a very nice card at the same price (at the time) of a stock 7900gt. Just limited availability though. They came out a bit before, and much cheaper, than the 7950gts with 512mb
Finally, MSI which does not have a lifetime warranty, but a pretty painless RMA process as well, replaced my x1900xtx to an x1950xtx when I got it over the summer (don't remember when, well before the 8800s came out). Can't complain with any of them overall. Especially since I got the x1900xtx at $300 in the first place.
