The Multigrade Paper developer will probably give you very similar results at the two dilutions. The two alternatives are provided in order to provide a choice of development times, and to offer the ability to economize if you habitually don't use to capacity the amount you mix - in essence you waste less if you discard developer at the end of a session.
The contrast difference Mick refers to can occur, but is most likely quite small with the developer you are using.
Fixer is designed to be used at the stronger dilution with film. Usually film fixer should be re-used. With paper, some prefer to use it more dilute, and for longer times. Other prefer to use it at the stronger dilution, for shorter times. As it is harder to re-use paper fixer reliably, and as used fixer is best handled specially (not discarded down the drain) the question of which dilution to use is essentially one of practicality.
If you don't over-use the fixer, and if you fix for long enough, there should be no quality difference in the results obtained from the two choices.
FWIW, I use the weaker dilution for RC prints, track my usage, rarely re-use my fixer unless I am again printing in the next day or so and take the used fixer to my friends who are recovering silver.
For film, I use the stronger dilution, re-use fixer, carefully track usage and do clip tests to confirm activity. My friends get this used fixer too.
For clarity, while I mix my film fixer and paper fixer from the same bottle of concentrate, it is important to mix up separate amounts of working solution for film, and working solution for prints. It is best not to use used film fixer later for prints, or to use used print fixer for film.
I am using Kodak Rapid Fixer currently, but I've used Ilford Rapid Fixer and Ilford Hypam Fixer the same way.