If you're looking for any kind of sustain, wackers are *not* the way to go.
You can get a good sound out of them for slap, if that's your style. You haven't really said at this point what you want them for, but I'll assume if you're trying wackers you're looking for an old school, rockabilly, western swing, etc. sort of sound.
I've heard about FMI's "pro" grade of wackers before, but no information about exactly how they're higher tension...
Speaking about weedwackers in general, a couple of things to keep in mind:
Let's just assume you don't want to play (modern) jazz, or any arco on them. They'd be crap for that. Period.
Get the black ones. Black dye has a higher molecular weight, usually requires the string to be in the dye bath longer - and so results in a (marginally) tighter string.
They take a long time to stretch/break in. Much longer than gut strings even. Like any string that costs $5 in material costs and is made out of landscaping industry materials, you can't approach them as if they should be performing the way you want them to. You might need to spend some time finessing a sound out of them.
But, if you can be patient with them, and spend a few months slapping the crap out of them to break them in - you might find them okay.
Try working on your slap technique. Pull *just* hard enough to get a defined note - and play some stuff you'd normally play pizz on to work on that sound.
If you wind up not getting on with the E and A, something dark like an orchestral steel string (bumped over even) should work well.
