Of the two the 7 Day is my favourite (in fact one of my favourite watches). It definitely wears quite big but does have the in-house movement.
The chrono (unless I am mistaken) still has a 7750-based ebauche though heavily reworked by IWC. I'd plump for the 7 Day though appreciate the price differential.
As far as replacement straps go I have struggled to justify the cost of genuine IWC items but have found a great source of aftermarket straps here: http://stores.ebay.co.uk/TimePieceRe...=p4634.c0.m322
I have bought a number for my 3717 Spitfire Chrono and they replicate the originals very well and have lasted well. They offer some genuine croc/gator but most are leather with an applied pattern. Not sure if they do the curved ends you require though.