How can the 4200 and 3500 both be 85 Ah?

Because amp hours and power are different measurements. Capacity is measured drawing a small current for 20 hours; power is measured drawing hard for minutes. CSB builds both blocks in the same case on the same capacity and configures the plates differently, so the 4200 delivers 370 watts per cell at 15 minutes where the 3500 gives 318. If you specify from the capacity column you will treat these two as interchangeable, and on a UPS they are not.

Which figure should I size from?

The watts per cell at the duration you need to ride, then the number of cells in the string. A UPS discharges hard and briefly, and the amp hour rating is measured under conditions nothing in a UPS cabinet resembles. Use capacity to sanity-check against a competing part, and use watts per cell to decide.

Is the 4200 a drop-in for a 3500?

Physically yes. Same 10.28 by 6.63 inch case, same 8.41 inch height, same I2 insert and the same torque, so the shelf and the cable lugs are unchanged. The shelf has to take 5.31 more pounds per block, and the charger has to allow up to 35.00 amps rather than 29.20. Confirm those two and the swap is straightforward.

What is the 30 second rating for?

Motor inrush and the first instant of a transfer, where the load spikes well above its steady draw. CSB publishes 980 watts per cell at 30 seconds for the 4200. If your load has a large inrush component, that column matters as much as the 15 minute one, and it is a figure the 3500 does not publish at all.