I asked the same question last year.
The 250 has a flawless track record, I bought it based on that fact over the 336FR. Don't let the 336FR's plastic cover between the clutch housing and the handle bars fool you into thinking its a true 4 point AV system, its not. Look at the IPL, it's just a plastic cover, although smaller husky trimmer are popular with the lawn guys for their "1 point" AV. The 535RXT has a two rubber buffers, one at the clutch and one under the handle bars.
If your budget dictates, I would opt for the FS310, Ive never run a 4 mix, but solely on the 4 point AV, better gear box and drive tube. If you want a 2 stroke, Save up for the FS360. I run my 250 without the guard and .105 line and and the vibes are no fun at all. I cant emphasize enough the AV.
On the note of the gear box, I have broken two gear box's now with my 250, not catastrophic failures from over powering (the FR480 powers the same gear box), but the "weed-winding skirt" is not meant for axial loading (down movement with the blade) and can crack around the edge. If you catch it in time, you can grind of the balance of the skirt and the crack wont spread to the gear housing. The first time I did this was with a three point brush knife, the second time with a shredder blade. Hit a rock or a larger stump, the blade bends upwards and game over.
The FS310 and upwards have a larger drive tube, more rigidity and doesn't "wallow' when you have the blade buried.
So I have 1.5 years on my FS250, my only gripes are the vibes and the breaking gearboxes. A true clearing saw solves these problems. My regret is that I could have bought a FS350 as they were being discontinued when I bought the 250. As of lately I have been buying FS350 parts to convert my machine to the one I should have bought in the first place.
All the best.