The O.S engine's are very good and reliable. They are easy to tune and hold the tune very well. CVery reliable and have a smooth power band. I would recomend the O.S engine. The .18cvr will have plenty of power more then the 2.5. The 2.5 is a .15. The 2.5 is in cc's and the .15 is in cubic inches and thats the difference. But the .18 will give you less runtime then a .12. You could get a .12cvr which has a lot of power and will have much longer run times then the .18. Both engine will be good choices it's wheather you want an engine with insane power or a engine that still has power but will have much longer run times.