Shimano external BBs (mtb and road/cross)

I have a couple of questions on the current range of external bottom brackets from Shimano.

First of all, the Deore/LX (M530) version is currently £19.99 on CRC while the XT/STX (M770/660) one is £43.95 - both with 20% discount already deducted. Why the extra cost, and is the XT version worth the premium? Is eg the sealing on the XT version much better or something? Rather confusingly they currently list XTR M970 at £39.99!

Also, would I need to use an MTB or road BB on a 'cross frame? The frame in question os a Giant TCX with 68mm shell, and the crank is an FSA Gossamer Compact. Is the spindle length on a road crank typically shorter so that only a 68mm BB should be used (ignoring Italian threaded frames for now)? And in any case what level BB should use for 'cross? Is Tiagra/105 any more vulnerable to mud than Ultegra/Dura Ace?

Thanks in advance,
Gareth.

Maybe not a lot of help , but.. I ran the external ultegra BB cups on a XT hollowtech crank for nearly a year through the sandy/gritty/muddy stuff we get up here in northumberland and it was spot on.. and cheaper only reason I dont have it now is that I sold the crank and BB

The new M770 XT version is slightly improved compared to the previous M760 one.

LX is also compatible and there's as near as no difference between the two in terms of durability and smoothness.

Road versions don't use spacers so no possibility to adjust chainline or fit 73m BB shells. Otherwise same technology.

Re. Cross - don't use Shimano BB as it might not work right with chainset. FSA have a multitiude of specifications for their various models so check out which BB they recommend for your specific chainset.