The Titan PowrLiner 850 and PowrLiner 550 are so close to being the same, that it's hard to distinguish between the two and at first glance, price is the only thing that says one is better than the other.
There are a handful of key differences between the PowrLiner 850 vs the 550 that we feel is worth the extra money, or savings, depending on which direction you plan to go.
Rotating Front Wheel
The first difference between the PowrLiner 850 and PowrLiner 550, is a rotating front wheel that allows you to keep all three wheels on the ground while you paint arcs and circles. The 850 has a 360º swivel caster as the front wheel allowing you to turn the unit with ease. In addition, the front wheel on the 850 is also lockable so you can do straight lines, giving you the best of both worlds.
Compare this feature to the Titan PowrLiner 550, and you'll find that the 550 has a fixed front wheel that doesn't move. If you need to make arcs or circles, you'll have to push on the handle bars to lift the front wheel. If straight is your angle, then the 550 is perfect for you. However if you need the ease of making arcs or circles, you're better off spending the extra on the PowrLiner 850.
Detachable Spray Gun
The second thing the PowrLiner 850 does that the 550 doesn't do, is allow you to easily paint stencils and curbs. You can detatch the spray gun from the striping unit and have roughly 25' of hose to move freely around the unit. If stencils or curbs are your money makers, then you'll definitely want to spend the little extra for the 850.
PSI Variable Pressure Output
The PowrLiner 850 comes with a pressure valve that allows you to adjust the pressure down to around 400 PSI or up to the maximum of 3000 PSI. Being able to adjust the pressure will let you use the PL850 on grass fields without worrying about damaging the grass, or you can just as easily use it on pavement surfaces. The PowrLiner 550 has a static PSI output of 3000 PSI making it an "all or nothing" striping machine that is only good for pavement.
Piston Auto Oiler
One additional feature is that the PowrLiner 850 has is an auto oiling piston setup. There is a small reservoir you can fill up with piston-lube, then you press a button to get a shot of oil to lube up the piston. The PowrLiner 550 doesn't have this option.