the difference IS minimal.....and in MOST cases you'd be fine to use either/or.....but, if there's a tight bolt that you need to loosen or vice versa....then you can EASILY round on the wrong size hex...especially if you use ball drivers.
now, me personally, i bought dynamite and e-flite hex drivers...and they worked great. Then i started getting more into racing instead of just bashing...and i bought a set of Losi pro wrenches...metric, sae, and nut drivers in sae and metric...and man what a difference! i did get annoyed at how the drivers go up and down in size though...so i bought a set of EDS metrics since i use them the most..and i got the ball tip in them. Then i moved on to screw drivers and arm reamers...went all hudy. Still looking for a body reamer i like....
what i'm getting at...is not all tools are the same. they might be of the same quality...but handle material, bit lenght, ball or machiened tip...it's all different per manufacture. I used a lot of them, and mixed and matched to make the set i have now, and i'll NEVER have to buy a new set again now! good tools make any job more enjoyable, as long as you enjoy the tool. so try some out, and when you're ready to get GOOD ones....do it. but any of the cheap $25 sets will work until you find the right ones.