What do I need for replacement

Looks like you have either the S351-23 or S390-23. The only difference between these two balances is the depth of the bottom nylon shoe, so compare both with your old hardware to see what matches best. For a new stamp option choose 2240. 

If by side clips you mean take-out clips, see our 16-002. 

Each pair of balances should be identical for the sash they're used with. The sashes in the same window may indeed use the exact same length and stamp options, but the sashes also might be different sizes, in which case the balances would differ. You'll need to inspect them to see. You want to replace the stamp options the original pairs used. Unless the window had trouble functioning to begin with, using new stamp options would not work out well and result in windows that are too difficult to either open or close.