I have tried the "low iron" glass that transmits a higher percentage of UV called Starfire but in the end got better results and a much better price with photocopiers glass. Photocopier glass is not cheap to be sure but if you are not in a hurry and call around a bit you can pickup some discontinued or "new" old stock, the price is much more reasonable. Mine has a couple of mounting holes in it but I didn't bother cutting them out as the glass size allowed me to print 11x14 with plenty of margins (containing the mounting holes).

If you know of a printing company (almost non-existent now) or an organization such as a fine art intaglio print studio that has a light integrator and a UV plate burner you can take a small sample (1" x 1") of the glasses in question and place it directly over the integrator measuring cell and with the burner properly (consistently) warmed up, determine how much more effective the glasses are relative to one another. This data can help you make the determination if the cost of upgrading is worthwhile.

Perhaps the ultimate contact printing frame is the commercial printing vacuum frame setup that sits under a plate burner. High efficiency UV glass that is large and the vacuum keeps the paper flat. If you're lucky it will still have the matching burner and integrator as well and this will make printing a pleasure to do consistently. The down side is that this setup is quite big and heavy with a storage cabinet underneath with a compartment containing the vacuum pump (will it go down your basement stairs?). Somewhat expensive even at a fraction of the original price ($100s instead of $1000s) But if you've got the room you might find one languishing in some reclaimed equipment depot. Getting somewhat scare now as everyone has switched to digital.