Maldives or Sipadan...

I think you should find a way to go to Sipadan if that's what you've got your heart set on. There's nothing like thinking you've 'settled for second best' to diminish your enjoyment of a dive destination that otherwise would be great.

I've dived the Maldives twice and Sipadan a number of times. I personally prefer Sipadan. There's just nothing to do in the Maldives other than play in the water and sit around in the sun. I nearly went nuts on an island-based resort during my first visit there because there were just two boat dives available per day and after a while the house reef became so familiar that it didn't excite me any longer. My second trip out there was on a LOB, which was better, in good part because it was all diving and not much sitting around waiting to dive.

You will not see a whole pack of turtles in the Maldives like you see them at Sipadan. There's no question about that. But based on my experience, you'll have a better chance at seeing sharks in the Maldives. Overall, I think the marine life at Sipadan is more prolific than that in the Maldives.

I completely agree with what quero says although I'm happy to be diving with the fishes no matter where. In the Maldives, I managed to squeeze in a lot of diving (in addition to the two boat dives) going around the house reef with my buddy and just studying fish interactions, but otherwise we would have gotten pretty restless. Plus, you say that you found a good deal on the flight and accommodation (like we did) but make sure that you don't have to add in drinks (even water) and diving costs which add up to quite a bit (at least around 500 euros per person I think). If you do make it to Sipadan, try to spend at least some time diving around Mabul too and not all around Sipadan. Either way, if you're not just focused on seeing one thing, I'm sure you'll enjoy the diving in either place.

Cheers

Hey, thanks to both of you.

I am sure I will love the Maldives - but am a bit worried about there being nothing else to do - not really used to that - I like exploring and finding out about the place I am in.

I think it all comes down to money really - I need to do my sums and make a decision - it's only a few weeks away!

I have no Sipadan experience, but did did the Maldives a couple of months back, so I'll add a couple of comments on that. I enjoyed my dives but for the cost to 'wow' factor, it might be a while before I return:
In addition to your flight costs you should consider 1) expenses on the Maldives (dives/hotels/food) are very steep (probably more the Sipidan?). Yes my hotel actually charged $12 for a bottle of water! (no alternatives on a one resort island atoll...) 2) location of hotel and proximity to the good dive sites (seasonal).
In the Maldives the best dive sites can be quite remote (think: expensive seaplane transfer to the atoll area and many expensive resorts..). If you have a 'good deal' on a hotel, check its location and the feasibility of dive trips from there. There are indeed cheaper hotels, but make sure the location will deliver the experience you want (a deal is only a deal if its near the diving), the Maldives has few truly budget choices.
I did some cool dives (saw sharks, rays turtles), but it burnt a (enjoyable) hole in my wallet

I've been diving both and Sipadan twice, I reckon there's nothing you will find in the Maldives you won't find in Sioadan while Sipadan offer so much more than the Maldives... first real people, a genuine culture ( I should say cultures if we speak about Borneo), then more marine life than you woul ddream of. Sipadan/Mabul is the only area I know where I spotted in one day a manta ray, a hammerhead, a school of housands of jack fish a school of hundred of barracudas probably over 50 turtles a few frogfish and a couple of pygmy seahorse (yes you read me : just on one day... i should have played cards this day) .

Go for Sipadan, it's a legend in a diver's life. You'll never regret your money spent. Guaranteed.
Don't forget it's monsoon time in the Maldives, you may also experience poor viz and some harsh currents.

You also might consider the information in this thread .

Go to Sipadan, never been to Maledives.
If you stay on Mabul you can also get very cheap accomodation at a longhouse homestay..