According to the latest sds sheets from Ilford:

http://www.ilford.com/html/us_english/sds/sds.asp

There are three very minor differences between Hypam Fixer and Ilford Rapid Fixer.

1. Hypam Fixer contains some Sodium Acetate (the sodium salt of acetic acid) the Sodium Acetate would tend to act as a pH buffer for the Acetic acid.
Ilford Rapid Fixer does not contain Sodium Acetate.
2. Hypam Fixer contains some Boric Acid, Ilford Rapid Fixer does not.
3. The pH of the Hypam Fixer concentrate is listed as 5.1 The pH of the Ilford Rapid Fixer concentrate is listed as 5

Sounds to me like these two acid fixing concentrates should function pretty much the same. Buy the cheapest one - or buy some 60% Ammonium Thiosulfate instead.