Having already had an SMP myself and now having a P* I can say HPA I'd good mate but for self builds like this it can be very expensive. My SMP cost me an absolute fortune and I constantly had barrel/hop issues.
If you buy either system then do testing before changing out the internals. Example, alignment checks with the nozzle and hop and fire testing to check range and accuracy, do this whilst the weapon is still electric then once you've changed it out you can look for any differences. You also want to put it in a GG& or VFC body as people have had better results. G&G is also good but not all G&G bodies work great with these systems.
Other issues with HPA is sites allowing them and other players hating the ROF these put out, theres a fair few HPA owners who have ROF set stupidly high. Also you need to fill you bottle/s' as well
Personally if your looking to get into HPA but not spend LOADS of money then get a Tippmann M4 they are excellent. If you need any help or advice Pm me I have plenty of knowledge on these systems :-)