Prehistoric spiral sash balances

You aren't kidding about the age of this balance. As I'm sure you've already guessed, this exact design hasn't been made in a long time. The good news is that we sell modern parts that work just as well, if not better.

See our Series 500 spiral balance for non-tilt windows, along with our 17-012 bottom bracket (sold separately). We've been using this combination to repair old wood windows like this for years. Plus they have metal tubes, not plastic, so they'll hold up a LOT better under sun exposure.

At that point, both options would work equally well. A stronger spring option doesn't always mean better operation; keep in mind that if a spring is too strong then you'll actually have a harder time working the window. However in this particular case either one is perfectly fine.