well, on a 120-140mm steel hardtail frame, you would like the strength of a bigger axle, as it enhances the ride, after i went to 20mm from QR, i can tell you it makes a big difference. on bumpy stuff and turns it remains more controlled, feels stiffer and is more stable. i remember having the wheel flex until it rubbed on my forks with quick release.
if youve got a good pair of wheels already, you can still get good qr forks, and that would be a good setup. it depends what sort of thing your riding, – for xc use, qr is fine, for more downhill/all mountain/jumping then i think you would be better with bolt through
so in my opinion, its worth geting 15/20mm. in your case i think the best plan of action would be to just put the 100mm forks on that you have, ride it like that whilst saving for some 20mm forks and wheel.
i hope this helps