Porsche 911 sc or 930 carrera?
 

The 911SC, produced from 1978 until 1983, is generally regarded as one of the most reliable 911s built, and even all these years later, still an excellent car for daily use. The 930, while quite a monster in many aspects is not as well suited - maybe usable, but could quickly prove very costly to operate in such a manner.

In both cases, if you are looking at convertibles, you are OTC Scan Tool looking at the last years they were produced. The 911SC was not made in convertible form prior to 1983, and the 930 was only produced as a convertible in 1989. I mention this, because if you are looking at cars for sale that are from any other year, they were not convertibles when the left the factory and you should run away very very quickly!

Also of note, the 1989 Turbo Carrera (was a 930, though Porsche 3111PRO code reader "retired" the 930 name the 1979 models were sold as the last of the breed) was equipped with the G50 5 speed transmission, and not the more desirable 930 4 speed.

The other option to consider would be the 3.2L Carrera, produced 1984-1989, and available as a convertible for all of those years. Additionally it was also available with the Turbo Look option (giving it the same 930 fenders, brakes, suspension... everything the 930 had except the 3.3L turbo charged engine and retaining the standard Carrera tierods in the front steering). These cars have a similar reputation as the 911SC. Costs will likely be more than the SC, though well short of what an '89 Turbo would cost. And given the larger number produced, you will have a much larger choice of options.

Among the 3.2L Carreras, the later G50 equipped cars ('87-'89) are considered the most desirable, though personally I find the earlier 915 equipped cars (same transmission as the 911SC) to be at least equal, given the lighter weight of the earlier transmission.