The barrel on the Troy PAR is threaded for muzzle devices, I've seen some complaints online so it is possibly threaded in a less common pattern than most. Mine came with the Troy "Medieval" muzzle brake, and it works great so I see no need to change it.
I own both an M305 and a .308 PAR. At this point I love them both and wouldn't want to trade either of them.
Accuracy: The PAR will be more accurate than the M305 out of the box. Most people are finding their stock Troy to be pretty close to half-moa. Norinco M305 will be more in the range of 2+. My M305 is around 1.5, but it was my Grandpa's and I'm sure there was some tinkering done to it. My older brother's is stock and somewhere around 2.5+, and my younger brother spent a pile of change on his to get it down around 1ish+ moa.
Reliability: Shouldn't have issues with either. The M305 is a proven bulletproof semi-design, and the PAR is a well designed but simple pump design that is based on the AR action and that takes some AR parts.
Fun to shoot: Purely subjective. Pumps are fun to shoot because working and action and being involved in your firearm's operation is fun. Semi's are fun cause mag dump bang bang bang bang bang bang bang bang bang bang.
Weight: PAR is lighter. But not by much. Where it has the advantage is in weight distribution and ergonomics, so the PAR feels much lighter than the M305 does. The M305 feels a lot less maneuverable and bulky in your hands.
Tweaking: PAR takes a lot of standard AR accessories, and as such is pretty easy to customize. It does come with quality accessories but it leaves you desiring a foregrip for the pump and the Troy stock is kinda ugly in my opinion. The M305 will likely need a bunch of work just to get it somewhat accurate and operating smoothly. There is a big aftermarket for them though so you can customize to your hearts content.
The quality of parts, fit, and finish will be nicer on the Troy. But the price tag will be much higher. You do however get what you pay for. The other advantage is that it is not a semi, and comes stock NR with a 16" barrel.
The M305 is affordable, fun to shoot, and with a little work will do anything you ask of it.
If I could only have one, I'd take the Troy. Also opens you up to the possibility of picking up an M305 later... "but honey, this one is only $500!!!!"
I will put a video of my thoughts on the PAR up eventually, once I am "done" with the customization. Have a rail section to modify and install, and an Atlas bipod on order for it. Being a pump it isn't very friendly to bench shooting so a bipod should help.