Does anyone know if there is an inherit difference between 1/8" minijack and standard 1/4" jacks?
Should I be suspicious of the m audio fast track's 1/8" jack rather than the regular 1/4" for monitors and full sized headphones?
It's really just kind of a trade off.
The 1/8" is lighter and easier to manage. The 1/4" is more durable but bulkier. I don't think it will make much of a difference with monitors (I am assuming headphone type) as far as sound goes. I think on stage floor monitors you may want to go with durable. If you are talking about just home recording then there would be no problem either way.
As Nils says, they work just as well electrically. I would rather have a 3.5 mm (1/8 in) audio jack mounted on a printed circuit board, rather than a 1/4 in version, especially if the sleeve of the connector is not chassis mounted for strain relief, as is the case for many of today's consumer and even pro-sumer products. The smaller jack/plug combo puts less strain on the PCB. The smaller jack also takes up so much less area on that PCB.
I also agree with Nils, that if you mean sound reinforcement (performance) monitors, go with the 1/4 -- even that is considered a bit iffy in these days of speakeron connectors and the like. Desktop monitors are okay with the small jack.
-=tension & release=-
The other half of the argument that Gnease put forth is that if the headphones you want to use have a 1/4" plug (like mine), but the device has a 1/8" jack, then you have to use an adaptor. Now, that's not such a big deal, but the typical 1/4 to 1/8" adaptor is this oversized plug/jack combination that, combined with the 1/4" plug itself, puts a lot of strain on the tiny 1/8" jack. The two acceptable solutions are to use headphones with a 1/8" plug or find an adaptor that connects the 1/4" jack to the 1/8" plug with a short length of cable, reducing the strain on the 1/8" jack.
They're not so easy to find, but here's one.