You call a 512GB SSD a little drive?
You recommended the OP get a 256GB SSD or [1TB] Fusion Drive. Yes 1/4 the size is little relative to something 4x as big.
Now, 256GB may be all he will ever need for the life of that iMac. If so, by all means get the SSD. It's the same price as a 1TB FD, and much faster.
SSD is probably more reliable, but SSD drives fail also, plus operational errors happen. If you're doing serious work it must be backed up, regardless of drive type.
However the two specific models the OP mentioned were neither SSD nor FD. Both have 1TB 7200 (non-Fusion) drives. I would recommend getting either 256GB SSD or Fusion 1TB, based on what meets his size and performance needs.
That said those are discounted prices, and you can't get a new iMac 27 with SSD or FD for anything close to those prices. At least it's a 7200 rpm drive, not a 5400 like on the iMac 21.
Also -- between the two he listed, one is apparently a 2012 model, since the WiFi specs only state 802.11n. The other one has 802.11ac, which was added in 2013. Also the CPU's are listed as 3rd and 4th generation, respectively, implying 2012 vs 2013 model years. In general I'd get the 2013 unit.
However if money is really tight, he'll be happy with either one. If he doesn't have 802.11ac WiFi in his vicinity, that's a non-issue. The smaller 512MB GTX-660M GPU vs the 1GB GT-755M is only a factor for some things. Both GPUs are internally based on the same Kepler GK107 architecture.