She'll get frustrated with the little stuff and you can use both in tandem later. Duplo definitely atm - Woolies , Argos, John lewis, The Entertainer, Mailorderexpress all worth a look for bargain buckets of it
I have been trying to get duplo on ebay but it seems to go for high prices and then they aren't always the pieces you want, there will always be an essential piece missing, so am just going to go and buy new from woollies or somewhere.
It was hard to get hold of in the UK, so I bought it from this website in the US. Although the postage was a bit hefty, given the current exchange rate I thought it was good value as dd has had so much playing time from it. Loads of people have asked where I got it from - it's a really unusual set, but perfect for age 2 upwards.
25% off lego in woolies at the moment (but might be the small stuff rather than duplo).
Most of ours is secondhand (car boot, charity shop etc) because it is indestructible and you can give it a good clean in the dishwasher or washing machine.
My dd (2.5) has the legoville duplo dolls house, which is about £30. It's fab and very pink. You get lots of pieces and can build a whole house complete with garden. If you are anywhere near Bluewater shopping centre they have a Lego shop and we've managed to get some bargains there.