I haven't purchased from either, although I have heard of them. I think they're both reputable, but somewhat overpriced, particularly Burt Associates. For comparison purposes, take a look at this vendor (one I've bought from many times):
www.bambu-u.com

i haven't bought from either one, have you tryed ? http://www.lewisbamboo.com/ i bought a grey henon from them and ive had good luck from it..

I have bought two plants from MAB. They arrived quickly and in great condition. The only problem was the Moso doesn't survive the winters here, it had complete topkill. If you're buying Moso I suggest "Hortus" on ebay, his are more cold hardy. I also bought a Vivax Aureocaulis from MAB and it's doing fine so far. I would definitely buy from them again, but I would make SURE what I buy there will work in my zone.

I have dealt with both, both are reputable.
One caveat, be sure the plants you select are appropriate for your growing zone.
I personally push my hardyness zone and as a result lose a few specimens which I can't in good conscience hold the vendor responsable for. You can push the zones if you get a couple mild winters to establish a rhizome mass-but the 1st year is very risky.

Over a period of three years I have purchased six bamboo plants from Bert Associates, four by way of mail order. All but one arrived looking vagarious and healthy. The one that did not look good soon died. I emailed him and he replaced it free.

I would recommend him.

If you need an interpreter for that post above, read "vigorous" for vagariousÂ.

I've purchased from Horseshoe Nursery for cold hardy varieties - great plants and reasonable prices.

Here is a link that might be useful: Horseshoe Nursery

I ordered a Phyllostachis nigra from MidAtlantic Bamboo about a year ago. The plant arrived promptly, in excellent condition, and has proved very vigorous. There would be no hesitation purchasing from them again and I probably will in the near future.