Russia - S7 or Aeroflot?

I've never been to Russia before and have to travel to Irkutsk in the middle of December. What would be the best airlines/routings to get there? My start point is Cambridgeshire in the UK, so any of BHX, STN, LTN, LHR or LGW would suit as departure points.

I've flown to Siberia on SU and S7 and in my personal experience both are absolutely fine. SU are SkyTeam and S7 are OneWorld, so that might inform your decision in terms of earning miles.

S7 are via DME, which for many years was considered the better airport, but SVO has improved a lot so connecting on SU needn't be a pain! S7 have a hub at IKT now I believe, so in case of problems there might be more options with them there (don't quote me on that!)

I guess it depends on what you mean in terms of "best routing" (seats? food and drink? IFE? connection time? etc)

I agree, they are fairly similar.
We generally try to avoid flying S7 since this (http://www.flyertalk.com/forum/travelbuzz/578989-irkutsk-air-disaster-personal-view.html).