In reply to le_quack:
As other folk have said SOL is much lighter and combined with a sleeping bag would be pretty effective (if you'll be with a mate consider the 2 man design - much warmer). If your sleeping bag gets soaked though - you've lost all your insulation. It's also more delicate and if you tear it, it'll shred it's self pretty quickly in windy conditions. I've got one for MMs and light weight days (or conversely bothy trips etc where I've loads of other warm stuff anyway).
Blizzard bag is heavy but hugely warm on it's own (much more so than the sol or a bivvy bag.) more like a sleeping bag than a bivvi bag! I carry one year round in Scotland normally. I've played a casualty in training exercises both with and without and it was a hugely different experience even in summer.
Downside is once it's out of it's vacuum pack, you'll need a spare rucksack to carry it as it expands a massive amount!!!
If I had a sleeping bag, and was expecting no more than a night out in bad weather at worst, I'd go with the SOL. If expecting (or threatened with) having to sit it out for multiple nights in bad weather the Blizzard.
Cheers,
Stu