I found Widows to be easier than Soldiers when soloing. The Widows Dart does the same damage as a Soldiers Heavy Burst, but cost less end and recharges faster. Widows have DDR (not much), get access to KB protection while Soldiers have no DDR and have no KB protection which takes 3 slots just to cover that hole.
I took Stealth from the concealment pool at 4th level for both. The Widow is able to run with sprint and ninja run 50-60% of the time. A solder can not, they'll be out of end after the first or second fight with a mob of only 3.
The devs should have made the Pain Tolerance power either a Superior Conditioning or Physical Perfection power instead. Either power would've fixed the end problem for either AT and allow a better recovery on HP and Endurance if you put a Power Transfer Heal Proc % and Performance Shifter Proc %.
HEAT's just need mez protection early on without having to force you to take Dwarf form for the protection at level 20 while Tanks, Scraps and brutes get it by level 12 I believe.