Matt,
A couple of guys here on TBN have discussed (extensively) the various tire options for their new M series tractors. Look for posts from Caledon Hills and Island Farmer. Caledon Hills has been able to work his tractor with optional tires - so, he's sure to have a good idea of how the different tire setup has benefitted him. Island Farmer has not had his tractor delivered just yet... (at least he's not communicated with the TBN folk's) so, he won't have a "real world" opinion just yet. But, he's certainly well versed with the different combinations as well as the prices.
A search for their posts and a PM to them might be a good first start.
AKfish