I don't have any direct experience on the 500 vs 900 debate, but I have owned both the 505 and the 905, FWIW - and the differences between the two lines are likely the same...
They seem to share the same pups and preamp, typically - although the 905 seems to be slightly clearer sounding in a band mix, yet has a warmer sound to it - for lack of a better description... Being a neck-thru, the 905 offers both top loading and string-thru options - which *to me* aren't any big deal... but, neck-thru's also typically have better upper fret access - which holds true here, too...
The 905 has a poly finished neck, the 505 doesn't - which may make a difference to some... As noted, the 905 has a "dished out" body - which I didn't think I was going to like, but actually did - it's very comfortable... They both appear to have the same bridge and tuners...
I've owned the 405, 505, 535, 655, 755, 905, and 3005 - and out of all of them, have kept the 905 - if that tells you anything... I would've kept the 3005, but flipped it for a profit to partially fund the purchase of an SR5 HH - it was a really nice instrument, but the 905 still gets the job done, and gives me a light 5er with a different sound than my SR5 - for around $500 used...
In short, if they're both in the same condition, I would pay an extra $100-$200 to go with the 900 - if it were me...
Hope this helps,
- georgestrings