I've seen both the Bubblegum Crisis OAV and (as much as has been released) of BGC T2040, and they're both great. Bubblegum Crisis Tokyo 2040, however, is slightly better in my opinion. 2040 follows a definite story arc, where the OAV felt kinda bitty. Plus, the characters in 2040 are a) better developed b) more interesting and c) both more and less extreme. Linna actually has more than a handful of lines and isn't the selfish person she was in the OAV, Nene is immature but funny instead of immature and annoying, Sylia is no longer cold and silent but TOTALLY insane, and Priss is no longer a bleeding-heart heroine but a cold and bitter mercenary. The character balance is better, the animation better, the lines more memorably funnier. One bad thing: the music isn't, in my opinion, as good in the dub of 2040, but I can stand to watch the dub of 2040, which I couldn't with the OAV. 2040 is a series where the sub or dub argument doesn't apply to me. MegaTokyo is also more interesting, with its mix of high-rent city centre area and the slumming underbelly of the city (Sodo Ward. Whereas MegaTokyo 2033 was rubble, Tokyo 2040 has rebuilt itself and integrated boomers into its culture, giving the entire series a better explanation and more plausibility. Many of the gaps in the OAV are filled in and explained here, ironing out some of the niggles that were a little difficult to ignore.
In general, this series rocks! The hard suits are more or less the same as the OAV, the music is different, the setting better but the characters are better and more interesting; in 2040 Priss steps aside and gives the others more screen time than the OAV. I give this 5 stars, which I feel it deserves, and from what I've seen, the series just gets bigger and better from here!