A gallon of water is a little over 8 lbs (8.3 IIRC). A 100 gallon tank would be holding up to 830 lbs at a minimum (presuming it's filled up).
Rocks are heavier than water, thus they sink, but they are also displacing water. For 150lbs of rock, it'd really be like adding 150lbs - (the weight of the water displaced) and not really an additional 150lbs on top of where you'd started.
Another positive of clay pots is that they don't displace as much water. If you had a 100 gallon tank filed up with clay pots, perhaps you'd still have 80-85 gallons of water volume. If you filled up that tank with rocks, you might have 60-75 gallons of water volume (this is just meant as an example and would largely depend on what "filled up" was and with what kind of rocks (round river rock taking up more room than holey rock or lace rock).
I think it comes down to what you'd like to look at. For me, I like natural aquascaping and there is no large quantity of clay pots in the lakes where these fish come from, but there are rocks. So I choose rocks. But its your tank, so go with what you'll enjoy.
