Definitely make it Varadero - otherwise you lose a large chunk of holiday travelling back and forth to Guardalavaca.Also downtown V'dero is much livelier than Guardalavaca.
Iberostars in Varadero recommended - can't recommend hotel in Havana as have only visited on day trips.

The Seville would always be my first choice in Havana. Yes. it's a bit rundown and some of the rooms a bit shabby and, yes, the rooms can smell a bit musty when you first arrive (running the AC soon sorts that out) but it is a very traditional colonial building with a central open patio with bar ideally suited for a drink whilst listening to the live music which is usually on offer of an evening, couldn't be better placed for exploring Havana Vieja, is stone's throw from the Malecon, the old squares with their shady cafes and all the live music bars and it has a swimming pool which is a rarity in downtown Havana.

It's not for those who want all the accoutrements of a modern tourist hotel (you'll see if you read the reviews here that not everybody shares my opinion of its charms!) but it has atmosphere in spades and is very centrally placed. And that swimming pool is a godsend when it comes to having somewhere to chill and cool off in the late afternoon after a hot, hard day's sightseeing walking around the old town and before getting ready to go out for dinner.

I can't really help out re Varadero compared to Guardalavaca - beach holidays aren't really my thing - three nights in Varadero at the end of a tiring tour had me climbing the walls and wishing that I had gone straight back to Havana for a few days or else gone to somewhere really quiet like Cayo Levisa for a truly realxing time before the flgiht back. But it's worth bearing in mind that Varadero will be a shorter transfer to Havana and as brewerdave says, it's an expensive taxi ride into any of the nearby towns with any sort of nightlife from Guardalavaca. But bear in mind that Varadero is a very long peninsula and some of the hotels are also some distance from any non-hotel based nightlife. Also, what is on offer in the town is very much aimed at the tourists with not necessarily that many opportunities to experience the sort of bars and music venues that the locals will frequent.

SM

Thanks to both of you, I appreciate your comments. The Seville sounds like our kind of place, even before I look it up. We really will need to be close to some kind on nightlife in Varedero, however non-authentic so we're looking at Melia Las Antillas.

If I can help with any further info on Havana let me know - and that should have been the SevillA :-)

SM