OK, I'll bite. Everybody's got their opinions, and they're all valid, let me add. Both engines have distinct advantages...
1. For the purist, an OHV mower engine is the only way to go!
2. For the cutting edge racer ready to really push some limits, a horizontal engine is the only way.
3. You can take your CP engine and throw it on an FXS/T class roller and go race.
4. Parts are cheaper for the horizontal engines.
5. Neither one lives any longer than the other.
6. Either way you may run into difficulties obtaining parts at the track if you were to get in a bind.
7. I would not recommend building just one engine. Drag an identical spare with you. Try not to use it until finals.
8. Having built a 33 for FXS, I love the torque it produced, but it was also on alky with a giant Tilly on the side. Another limiting factor is you can get 3,000+ more RPM out of a horizontal than an OHV due to the inherent limitations of the crankshaft.
9. A GX390 will only get better through the years, as new designs for heads, valve technology and supply vs. demand continue to take effect. The aftermarket loves these engines, as well as its clone version.