Well sand will make your final object much heavier and it won't really be hypertufa. The sand will also give you a much smoother finish. The perlite will give you a much lighter finished product, but the finish will be more porous.

I have been in some heated exchanges about tufa and what it is. My conclusion is: a bunch of folk who make hypertufa continue to demand steadfastly what the ingredients must be. BUT at the very least the mix must contain peat moss.
Naturally I disagree but rather than continue a close-minded discussion that will go nowhere I simply DONÂT make hypertufa. My pieces are made using just about everything; vermiculite, perlite, sand (both fine and course), latex paint instead of water, metal filings, polystyrene beads (bean-bag beads), anything, BUT I DON"T USE PEAT MOSS! Oh yeah, I will never use fibers that nust be "burned-off" when the project is finished.
Keep in mind if you want to "work" the finish, sand will ruin files and metal tools.
IMO use whatever you want to make your project a success and then call it hypertufa, hyperthreefer, or stretch it to be yaeltufa. Most importantly have fun and experiment.

Please note: Hypertufa From Wikipedia, the free encyclopedia
Hypertufa is an anthropic rock MADE FROM VARIOUS aggregates bonded together using Portland cement.
Hypertufa is intended as a manufactured substitute for natural tufa, which is a slowly precipitated limestone rock; being very porous, it is favorable for plant growth.
Hypertufa is popular for making garden ornaments, pots and land forms. Hypertufa is relatively light compared with terra cotta or concrete and can withstand harsh winters, at least down to -30 °C (-20 °F).
Basic recipe
Using the classic proportions for mortar (1 part cement: 3 parts aggregate) hypertufa is composed of
3 parts Portland cement - Type I
4 parts peat
5 parts perlite
(3 parts cement: 9 parts aggregate) Â 1:3 ratio

THIS SECTION SHOULD ANSWER YOUR QUESTION ABOUT USING SAND,
Other recipes may call for sand or gravel as aggregates, but they add unwanted weight without necessarily adding strength. To increase structural strength and longevity, synthetic reinforcing fibers (plastics, not fiberglass) and liquid acrylic may be incorporated into the mixture. Powdered mineral colors (in small amounts) tint the hypertufa to resemble natural rock.

Final words. Have fun, experiment, and expand your creativity.

Robert