Have been to both places and the two are really nice. Don't know if you have visited the area before but from both you cannot walk to beach. You need to take a boat or at some places you can walk - its a good 10/15 mins walk. Beaches are all fabulous, blue flag, unspoilt and uncrowded.
In terms of the places Tavira is a small town with the river Gilao running through it with a roman bridge aross it, this is my personal favourite. It has tourists but is a real town too. There are plenty of lovely little cafes and great restaurants here. Cabanas de Tavira is about 2 miles away and is equally beautiful but more of a tourist resort only, although it also has great character and lots of cafes and restaurants too.
First went to this area 20 years ago so know it pretty well, at that time there only Portugaese there. Would say its still unspoilt and if you want an even quieter area check Santa Luzia, its a really small fishing village outside Tavira.
Hope this helps and enjoy holidays.