In my humble opinion if you buy or lease either a TU jet or to a lesser degree a Comet statistically you are doomed. Someone please enlighten me to the contrary.
The Comet 4's are, IMHO, among the better AWS aircraft (for the time period) IF used appropriately. Statistically, they are inferior to most 50's era props, but they are the first true jet on the market and the 4 variants offer range not matched in a comparable plane until the 727-200 Advanced rolls out. The 4 variant also offers similar fuel/asm as the 707's and DC-8's, so while thirsty they are not -that- bad.
If you order them, have an iron clad exit strategy (replace them with 727, 737, BAC's, or a mix), keep them flying as much as possible, and watch for that second D check, it gets pricey.