In my life I used to spend a lot of time visiting and spotting in Lisbon Airport! Lisbon is the place to see regular airlines like AF, SN, BA etc. etc. You also get some african and brazilian airlines! Besides that you have TWA / Continental flying their 757, and you can also see TAP fleet and their A319/320, A310-300, A340-300 and B. 737-300. Also you might see some portuguese charter airlines like SATA ( from Azores ) and L1011-500 from Air Zarco. Besides that you can always see some very rare cargo airplanes but you must be lucky! I moved 2 years ago to Faro, and at the beggining it was great for me, most of airlines I didn't even knew, but now I becoming aware how the thing goes! Monday its Holland day, you'll see many Martinair , Air Holland, Transavia etc. Tuesday its Scandinavian day, many Premiair, Blue Scandinavia, Finnair, Braatens, etc. Wednesday its German day, that is a very buzzy day with lots of german airlines, Hapag-Lloyd, Germania, Condor, LTU etc. etc. Thurday is British day but... you don't want to know that yes? Friday its a little bit of everything, Saturday its IL62 days and some mixture of all the others and Sunday the same too! So my advice is, if you have time to get both its would be great! Facilities to spot in Lisbon and Faro are excelent!
Have a great time!
CV990, the Maserati of the skies!