I'm grouping these together because I've used all of them extensively and they they share a lot of the same components and I will clarify the differences.
The HEART - All of these masks share the same heart, the spectra thermal lenses. They are a two pane design and in my experience are the BEST lenses when it comes to not fogging or building up condensation. Visibility is good, with an advertised 260 degree field of vision. The lenses are smaller than those on some other brands. Save Phace masks and some Vforce masks for example have a larger lense and offer slightly more periphery visibility. They are not the hardest lense out there either, and can scratch more easily than the Sava Phace masks.
Style - As always, style is opinion based. I loved the JT style ever since the spectra was released back in the day. It screams 'traditional paintball' and being an old school player, that suits me just fine. It's not as aggresive as the Grillz or as organic as the Invisions. It's pretty 'to the point' and imo, sharp looking. The Flex 8 breaks away from the others a little here, and is slighty thinner and the grill design on the mask slightly more styled than the Proflex and the Flex 7.
Customizablility - There is a lot you can do with these masks nowadays to really give them a unique feel. Different lenses(reflective, smoke, clear, amber, etc.), different goggle frame designs(camo, red bandanna, Union Jack, etc..), various straps(german flag, camo, flame, olive drab, ect..), goggle skins for the lenses, removable adjustable visors, soft or hard earpieces, ect... It's nice to have that mobility to customize. I took a black Proflex, swapped out the strap for a camo one and threw on a digicamo SkinZ on mine. There are a ton of options and that personization is, to me, really cool.
Maintenance - JT has never been known for easy goggle removal. The newer masks though, specifically the Proflex, are easier to swap out lenses than previous masks. Cleaning is simple, but can be time consuming as there are a lot of venting holes that paint can get into. JT provides cloth goggle bags with their masks for easy scratch free storage.
Comfort/Features - JT uses a very comfortable form fitting foam on their goggles, and the addition of a chin strap on the more recent models helps to fit the mask to any sized/shaped head. The masks(especially the proflex without the hard earpieces) are light in comparison with some other big brands. The proflex and Flex 7 has a soft rubber lower faceguard and a soft plastic visor. These are nice features as they do encourage bounces more than some other masks. I'm not the type of player to 'hope for bounces', but it is nice to once in a while feel one riccochet off your visor. My Scott Badboy mask and my Save Phace mask are both great masks, but cannot claim to bounce factor that the Flex can.
Ventilation - Breathing in the JT Flex series masks is great and imo, at the top in this category compared to other industry leaders. There are plenty of air holes to allow access to fresh air...nuff said. The lense frame also has a series of vents and holes to allow sweat evaporation and prevent condensation in the goggle. Very well designed.
Sound - Some masks can muffle the sounds around you and make it hard to communicate. These don't. All of the vent holes in the mask make it easy to project your voice out, and the earpieces don't act as earmuffs. Now there is a difference in these masks here. The proflex uses a soft(what I think is neoprene) earpiece where the flex 7 & 8 are hard plastic(note that these are interchageable). The hard plastice earpieces are great, provide awesome pretection and allow you to clearly hear whats going on around you. The soft ear covers are what I prefer. They are thick enough that a shot to the ear won't sting. These allow me to hear everything as if I wasn't wearing a mask at all. Very clear sounding - better than any other mask I have ever tried on.
Durability and Value - I've had the same flex 7 mask for about 4 years and it's been to hell and back and still is in excellent condition. The composits and rubbers they use are top notch. Good quality here! I bought my Proflex for about $65. To me, thats a steal as I know these won't fog, won't crack, and will last a long, long time.
Overall? I love JT masks, always have and it's becasue of the reasons above. I'm a big fan of Save Phace and Vforce as well, the Flex is just more 'me'. It's an excellent value cost-wise and it's imo the most comfortable option out there.
Proflex:
Flex 8:
Flex 7:
