I also was debating buying xEnviro due to the price. Let me tell you, as a real world aviator this product is the best ive seen. I do not regret the purchase one bit.
SMP4 is great too, however in my opinion xEnviro offers a whole environmental package that really leaves you feeling the challenges of flying from within the sim. things like glare and rain are real hazards while flying and xEnviro depicts this so very well.
Another very very happy customer. The enhancement is absolutely remarkable.