Depends on what you have to offer, really.
Mollies can be sensitive to soft, acid water- they prefer it hard and alkaline, and some even like it mildly brackish. If the water is not right they can be prone to disease. They are also bigger than guppies and therefore need more space. But if they get things their way they can be as tough as nails.
Guppies are not quite so fussy about the water, but some guppies are quite inbred and therefore not the strong fish they used to be. Look for big healthy-looking strong bodies.
Both fish mate at the slightest opportunity, any time of the year, give birth after 4-6 weeks and can store sperm from one mating for up to 7 batches of fry. Females may already be pregnant from the shop.
Both need to be kept at a ratio of 2-3 females per male (or single sex groups- though then you need more than just a couple of males). They benefit from a good varied diet, good quality flakes supplemented by vegetables once or twice a week, and the occasional treat of bloodworm or similar.
Both are very nice fish- curious and inquisitive and easily tamed.
