IMO which one depends on your water quality. I've sold a few thousand mechanical metered 5600s and some SE versions.
With 1000 of them sold to DIYers, I say a better valve is the Clack WS-1. No model of the 5600 comes close and you can repair a Clack WS-1 in much less time (total rebuild in under 30 minutes) and no special tools are required as with Fleck valves; just a pair of channel lock type pliers. The parts cost less too, by over 50% less.
So why the 5600? If because of its price, one service call or DIYer fix at the price for Fleck parts and you end up spending more than had you bought the Clack WS-1. The Clack WS-1 has the same piston seal and spacer design as Fleck valves and huge improvements in the design. It also has many other improvements and features that the Fleck 5600 and 2510 in any version.
How do you know you need a 1.5 cuft (48K) softener? You should beware, to get the max 45K regenerated capacity, you need to use 22.5 lbs/regeneration. That is terrible salt efficiency. Have you studied my sizing page or used my calculator?