The SP-15 and SP-25 are virtually identical tables mechanically, differing mainly in their electronic control and speed precision (and of course the 78RPM capability of the SP-15). Both tables have a very high-torque motor, and their bearing is of high quality, and very durable. I use the superb clamp available from Pro-Ject (Sumiko in the USA); it is VERY heavy, and doesn't "phase" the start-up time or rotational accuracy of my SP-25 at all. You may, if you want, desire to have the brake potentiometer re-calibrated to obtain the same stop-time, now that you have the extra weight of the clamp to deal with. Furthermore, it's not really a clamp, but rather just a heavy weight, as it does not clamp onto the spindle at all; it just sits on it. It is therefore extremely practical, easy and a pleasure to use on these tables (and for those lucky SP-10 owners as well). You should have no worry whatsoever about whether your SP-15/25 is durable enough for a heavy clamp. I have obtained superb results from it.
I hope this helps.
I hope this helps.