26" SERIES 385 CHANNEL BALANCE

Hi,
Which one will I order from here. Last time I ordered the 2510 but the spring was too weak. It did not last.
Can you help me choose from the sash weight range?
The channel balance has a stamp 25 5 AW CMC 2.

Stamp Option Sash Weight Range
2510 5 to 9 lb
2520 9 to 15 lb
2530 15 to 19 lb
2540 19 to 25 lb
2550 25 to 28 lb
2560 28 to 30 lb
2565 30 to 33 lb
2570 33 to 36 lb

Thanks,
Maria