I'd say the Algarve would be slightly warmer but there should be more going on in Benidorm! Should still be in the 20's too but you may get a downfall or two!

I'd go for the Algarve personally, I suspect the weather will be better and it should still be pretty busy especially in Albufeira old or new town

It depends what you want mari, if it's entertainment then Benidorm will win. Asuming you are thinking Albufeira, then that also is a great place with great beaches (Benidorm beaches are also very good), but entertainment a bit slower, there is the Strip of course, but not as much choice. More relaxed in the Algarve for me and I prefer the restaurants in the Algarve, but prefer the pubs/bars in Benidorm. Weather-wise, there should not be a lot in it, probably pointing more to the Algarve. There is more to do in one place in Benidorm, but as I always hire a car in the Algarve I tend to venture out a bit. Your spending money should go further in Benidorm though, lots of competition. Benidorm is bigger than Albufeira, but is also a "concrete jungle", but that's Benidorm - warts and all!!!
So what do you want from your holiday mari?

Robby

You do realise that the Algarve is not a resort but a region about 100 miles long west to east? The weather maybe pretty similar all the way along but the resorts vary a lot.

if i stay in the algrave it will be albuferia nearer to the old town i've seen some apartments think they are called casa mitchell apartments. will go to both places at some stage but going to corfu in june for 2 weeks and then one of the above for 1 week in sept. The other will have to wait until next year. It appears to be just as expensive to go for one week in september as 2 weeks in June to corfu.

Steve is correct, the Algarve is not a resort, but a region. Normally and as expected when comparing Benidorm with the Algarve it is fairly safe to assume they are thinking Albufeira.
mari, as you don't intend to go until late September, I would leave it late and possibly grab a late deal for either resort. The schools are back then and there must be a fair chance of grabbing a bargain, especially if the €/£ rate is still as it is now.
Let's know what you decide.

Robby

We went on a late deal last september to Praia do Vau and got 2 weeks for 900 euro and the weather was great.