The difference in Siphon & top loader is that the top loader is a bit easier to handle ( meaning :Better balance and control ) There are HVLP's in both models , and conventional in both type guns .
Most Paint manufactures recommend using an HVLP though , as thats where the technology is designed , but doesnt mean you can not spray todays paints with a conventional gun . ( you will just use more and have alot of overspray .)

The HVLP's models are designed for todays BC/CC materials for better transfer & less air pressure at the gun : Meaning more goes on the vehicle then in the air like the guns of old. but requires more CFM's to use an HVLP . Less VOC levels too .

There are many brands out there from High dollar to cheap throw-aways .

Hope this sheds some light for you .
Spray