Just cranked mine to over 800 miles on my 1200 (not got out much due to snow and ice arriving just after purchase)
The 1200 is a lot more stable platform than the 660 and wieght wise it belies its on paper specs, whilst it is fair bit heavier than the 660 it can make the 660 feel top heavy if caught on the hop .
Offroad the 660 should be easier to cope with picking up etc as weight will wear you down but its probably a bit more complex as the 1200 may actually be easier to ride due to lower CofG and better suspension stock.
The 660z can be improved but I found the stock sussies a bit short on springing and damping and if tempted into going as fast offroad as the bike appears capable of, it can quickly start getting messy with back end jumping about from full compression rebound and crashing forks (not apprarent when keeping speed down) buttock clenching at times.
On the road the 660 is a hoot but its still only a relatively moderately powered single so the 1200 has better capability for overtaking, with apparently amazing ABS and traction controls systems , though the 660 doesnt seem averse to a bit of backing in on the rear brake. Ultimately offroad both are a bit compromised by wieght and the need to run decent pressure to pinch flats at bay.

But I think the main reason for picking a 660 would be if costs were a big factor and you didnt want to carry the kitchen sink or pillion.
accessories for the 1200 are starting to come on stream but the manufactures seem to think prices can be tied to bike purchase cost !?

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ex XT660X Tenere, now XT1200Z Super Tenere