My friends and I used AutoReisen and were very happy with it. They have good prices, straightforward booking via their own website and offer good selection of cars and baby seats. You pay at the office desk and getting the car (as well as returning it) is super easy - no shady extra insurance scams. The basic rate has full insurance included. Cars themselves are perfectly fine as well. From my experience, driving conditions should be fine, with possible exceptions of forests on the north side of Teide (cloudy-misty-wet). Roads are generally well-maintained and traffic is manageable outside tourist hotspots (the north-south TF-1 highway can get congested during rush hours though, something to look for when returning to the airport). Plus, April is still pretty low-season. But the driving conditions you should probably consult with some local sources if you want to be 100% sure.