As far as the effects is has on your drivetrain, angles, etc, etc, it's really six of one or half a dozen of the other. They're both the same in that regard. The benefit from an aftermarket coil over that raises the front is you now get the added performance of new shocks. That's about it. As for detrimental effects, I think these leveling kits have been used for hundreds of thousands of miles by members on here with very little negative feedback. I say it comes down to money. Obviously the coil overs are going to be more than the spacer, but you get what you pay for.
Do some searching on this as this has been discussed ad nauseum. But I haven't heard too much about the Rancho's.
Bilstien vs Leveling kit
Bilstein 5100 vs AS 2" or 2.5"
'02 F150 XLT 4X4 SCrew. 5.4L
K&N FIPK/Amsoil Filter
Flowmaster 40 si/do Exhaust
305/70 16 GY MTR Kevlar
16X10 MB Motorsport wheels
Rancho 4" lift
Rancho 9000XL Pro Series Resi Shocks
FireStone Air Bags
Autometer Carbon Fiber Oil/Trans Temp Gauges
Carbon Fiber Interior (www.dash-kit.com
Peterson Fluid Sys. Remote Oil Filter
Troyer Performance Electric Fans
Amsoil Fluids Throughout