Dry treatments... great stuff verging on "no brainer" for winter routes, if you're not paying too much extra then "might use" is probably enough reason to get it. For plain rock I wouldn't particularly bother if you can get an otherwise similar without the dry treatment for less.
70m is a long rope... I've moved from 50 to 60 for that extra bit of faff factor on mountain routes but unless you're looking at routes where the extra is going to come in handy you could be carting around 20m of rope you don't need, so all the weight and bulk you save getting down to 9.4mm goes straight back again.
Which doesn't answer your question, of course! For me, I think I'd draw my useful length line at 60, though of course if it's a real steal you could always just cut the end off after a couple of years if you're not using the whole lot.
Pete.