Questions about Kicker KM speakers

I've been looking for some tower speakers and think I have narrowed it down to the Kicker KM 6.5's. I'll be putting 4 of them in DIY cans. I was looking at the KM620, but I'm needing to stay as low budget as I can. The difference I see between the 620 and 613 is the tweeter size (3/4,2 for 620's and 1/2,1.3 on 613's) and the Frequency R. (35-21 for 620's and 35-20 on 613's). My question is how much difference would that really be while riding? The 613's are about half the cost, so wondering if it's worth the tradeoff. Also thought about maybe 2 of each since the grills look the same. Had also been looking at the polk's since they're pretty common on this message board. Has anyone compared the two? Any help would be greatly appreciated. Thanks in advance.