If you are looking at either to purchase, my recommendation is always to buy the best you can afford thus the 400BX.
I have three D-Lite 2's, They are excellent entry level units. I don't shoot with modelling lights on as they are not fan cooled.
I recently purchased a 400BX, simply for the fan cooling, so that I could leave the modelling light on throughout the shoot. (assist focusing)
Testing between the two I would say they are as consistent as each other, the BX tactile keypad is a little more positive, and robust. The modelling light can also be altered in 1/10 increments independent of the flash setting, however, I don't see a use for that -not for me anyway.
The build is also a little more robust, and the modelling light is 150W of which I believe can be increased to 200W, whereas the D-Lite is 100W max.
In the US, the D-Lites are being supplied fan cooled, this might be the case in the UK sometime, but they haven't made their minds up yet.
Email from Elinchrom on 7th Feb 08 pasted below:
"Dear Stephen
Fan-cooled D-Lites are actually only shipped or will be shipped into
countries / areas where the units may suffer from high ambient temperatures.
It might be a serial feature in the future, but the decision has not been taken yet.
The D-Lite 100 is on hold for some time.
The actual line kept us very busy, so we had to change our original plans and we must focus on
production capacity and availability of components, since they are specific like quality flash tubes which are
difficult to make and capacities are limited.
I hope this helps a little.
Kind Regards
Thomas
Elinca S.A.
Thomas H.W. Beinke
Deputy Director / Marketing"
The D-lites are fantastic, and for an entry level kit can hold there own against some of the more expensive gear, the BX is a little better, but you will not see any difference in your images.
Good luck with your final choice.