As Georg has said, you will need some kind of transport to get to the best places at both airports. Spotting & photography from both terminals is pretty much useless.
If you have google earth, then here are some links to where I found to be good spots:
: Good views of landing traffic on Rwy 30. It's on a hill.
: Follow the road back North to see where to leave the motorway. From this spot, a dirt track runs parallel to the runway. It's the grey track to the South of the tarmac perimeter road.
Rwy 08 appears to be most in use at Tenerife South. I never got round to finding a good spot at that end last time I was there. However, a geocache
has recently appeared at the 08 end of the runway.
Airport - Tenerife Sur
Access road to cache site
It looks like there might be some good approach shots of landing traffic from both the North and South of the centreline from around here. Looking at the photographs
from the geocache website, it looks like open land where you can walk around to find a good place.
As the cache page says, Tuesday's and Friday's are the busiest.
Hope some of that is of use to you.
Flying is the second greatest thrill known to man... Landing is the first!