SS --- QB - Rep
35 agi 00 - 26
30 sta 31 - 00
62 Ap. 102 - 92
24 hit 00 - 19
00 crt 30 - 00
SS: 300HP, 97 AP, 1.5% hit, 0.88% crt
QB: 310HP, 102 AP, 0% hit, 1.36% crt
Rep: 0HP, 118 AP, 1.19% hit, 0.65% crt
So from the Sonic spear to Quantum: +10hp, +5Ap, -1.5% hit, +0.48% crit.
From the repspear to Quantum: +310hp, -16AP, -1.19% hit, +0.71% crit.
Simple conclusion: if you are hitcapped, Quantum would be good.
If you're not, getting either Sonic or the HFP repspear is better.
Unless you are really low on hp, in wich case the repspear wont do.
And both the repspear and the sonic spear scale with buffs because of agility.