As far as different aircraft models on a single type rating go, a LRJET type covers the Learjet 23, 24, 25, 28, 29, 31, 35, 36 and 55.

Because of their high fuel burn, loud engines, cost of updates (RVSM, TAWS) more and more 20 Series Lears are being parked. They burn the same on the ground at idle as at FL410. 30 and 50 series will be around for a long time to come. Lear airframes are not time limited.

If you like aircraft with great performance, go for Lear type. If you prefer some more comfort and are not in a great hurry, pick the CE-500 type.