Islands....
Fuertaventura
Great beaches, lots of wind, ideal for surfers. You might eventually die of boredom.

Gran Canaria
Nice island, similar in many respects to Tenerife. Playa Del Ingles and Puerto Rico are huge sprawling towns, very busy. To the south are a string of smaller towns, many nothing more than a few hotels in a bay. Puerto Mogan is probably the nicest of any of them as it has a nice beach, bars, restaurants and shops and is within easy bus distance of Playa Del Ingles/Puerto Rico.

Lanzarote
The most volcanic of the islands, resembles a moonscape in most places. Very little greenery or spectacular scenery, lots of beaches. Biggest resort is Puerto Del Carmen, big sprawling, busy resort, which is near to the capital Arrecife. Other are Costa Teguise and Playa Blanca. Nicest in my opinion is Playa Blanca. Lots of walking routes if you don't mind trekking over volcanic fields and mountains. Easy bus route to Puerto Del Carmen and the other towns. Ferry to Fuertaventura takes about 45 mins. Nice town with good selection of bars/restaurants.

I would vote for Playa Blanca in Lanzarote. I think it attracts more couples and families, not those wanting a clubbing type holiday. There are some good restaurants and the marina area is extremely attractive.
There is a very long paved promenade along the sea front from one end of the resort to the other for easy walking and the cliff top walk to the Papagayo beaches is not difficult though a bit up and down. The mountain behind Playa Blanca has also a walking path to the top.
Lots of places to visit either by car or as paid excursions.
As the previous poster stated don't expect spectacular scenery but I find the volcanic landscape very attractive in it's own way.

Judith

Playa Blanca for me too for the reasons Judith has stated.
The volcanic landscape on Lanzarote is fascinating and Timanfaya National Park and the Montanas del Fuego never bore me. The cliffs at Famara are spectacular as are the views from the road to Haria (Valley of 1000 palms). There are lots of great places and sights to visit. Hire a car, get a map and just point in one direction and drive. Some of the places you will come across are amazing.

Thanks all. Will have to give this a lot more thought!!

Been to Lanzarote twice and Gran Canaria once. Of the two much prefered Lanzarote, liked Playa Blanca, but stayed in Puerto Del Carmen and Costa Teguise. If I went back (hope I do one day) would stay at either Playa Balnca or Coasta Teguise again.

If you have a car the whole Island is yours and I also personally loved the 'moonscape' appearance of the island, when people talk about places not being scenic they tend to be talking about greenery, but I found the lack of this and the contrast to many other places facinating. Driving up high and disappearing into the clouds on an April morning was magical - and a bit scary.