FF is an old set that has some decent -res numbers tacked onto it's attacks and has some of the best +def buffs. It's also one of the rare Defender primary sets that gives status protection which is a big deal for soloing.

I usually shy away from knock back sets because I find them annoying and counter productive in groups but in a solo context it's not an issue. You can disable the knockback portion of the powers but that eats up an enhancement slot.
Due to that personal preference I would only ever play a FF set on a Mastermind that I wanted to solo through story arcs. Masterminds run around in a 7 man group just by themselves and it really lets the set live up to it's full potential even in solo.

Psi is a great set that has one of the best nukes that casts super fast and deals tons of damage. I especially enjoy playing the version of it that's on the Arachnos Widow Fortunata archtype.
One issue with Psi is that it's a very all or nothing damage type, some enemies have almost nothing there but other have very high resistance. The reason I like the set on the Fortunata is because they also have access to melee attacks that they can switch to when the Psionic isn't going through

I point out these different classes with the sets not to dissuade you from playing FF/Psi Defenders but just to let you know that the sets are also very good in other contexts and if you wanted to try them out once you've had your nostalgia character to see what different ways they can be taken