We have MSP gold on our Westwinds and we couldn't be happier.
The Westwind has TFE-731-3-1G engines. Ours used to have Electronic Engine Control (EEC) only. With MSP, Honeywell provided us with DEECs (Digital Electronic Engine Control).
We were told Honeywell wants to eventually replace all EECs with DEECs.
We checked with JSSI in the past and made the decision to stick with MSP. JSSI coverage was not 100% and did not cover things like loaner engines.
Also, something which had a lot of bearing on our decision was the fact that taking an aircraft off of MSP and putting it on JSSI has the tendency of reducing the resale value of the aircraft.
If our planes flew 1000 hours on JSSI and then we sold them, the buyer would be responsible for back paying for the 1000 hours if he/she wanted to put the aircraft back on MSP. In reality, as the seller we would end up eating that cost.
Suggestion is to go with MSP over JSSI, but I would contact both to get the full details about each program, specific to your needs and aircraft/engine type.