I left Eagle/Envoy in Jan 2014 after being there for almost 7 years at the time.
Having experienced both, I would likely go with Spirit. As you may know Spirit is in negotiations for a new contract. The current contract expired in August of 2015. Much of the hard negotiating is behind and Spirit and the union have been in mediation for over a year now. The company recently put out an offer that the union declined. A new contract will be signed, it's a matter of time.
That said, there are problems here, but even at current contract rates and even being in the right seat for 5 years, you will far surpass your Envoy earnings during an 8 year period when comparing the two even taking the Envoy retention bonuses into consideration.
If I read what's on these threads correctly, from date of hire at Envoy will be about 8 years till flow. If you are diligent in your job search, you should be able to land a legacy job in that time period. If not, the new Spirit contract rates will be near legacy rates so you haven't lost much unless you just value the prestige of working for a legacy.
With the above in consideration, QOL at Spirit also far surpasses Envoy. You will not suffer from lack of off time, you will not be force junior manned, no reserve turnbacks, 4 days off between blocks of reserve or sequences. All of these things combined make life quite tolerable.
There are great crews at Envoy. Many experienced professionals there so if you decide to go that route, you will have good company. Best of luck.