Thank you all for the advice. My initial instinct was to go for DL but their ongoing viability worried me.
The status matching does sound attractive as does the lounge access. My only question is that I thought Skyteam was similar to OW and *A - ie US airline FF elite members don't get lounge access on US domestic sectors, while elites from non-US airlines in the alliance do (eg AAdv members v. QF members flying US domestic segments). Hence my inclination for AF.
If that comment from graraps about AF F+ members with non-US addresses getting 100% miles bonus is correct then I see little advnatage of going with DL - I am unlikely to be flying DL domestically.
One additional question, how is availability of awards on alliance partners? are the redemption levels the same from any airline's program or are some better value?