In a contest between guppies and mollies, a tie is more than likely. The similarities between these fancy little swimmers far outweigh the differences. Ultimately, the only substantial difference in keeping each is the size of tank you'll need to get your fishy hobby off to a swimming start.
It's a Small World...
All guppies are one species, Poecilia reticulata, but they come in literally hundreds of breeds and color variations. There are a couple of different molly species, the most common being Poecilia sphenops and Poecilia latipinna, also available in many breeds and colors.
Notice a certain similarity? They're all the same genus, which means that guppies are as closely related to each species of molly as those mollies are to each other. Basically, when we're talking guppies and mollies, we're talking about the same general kind of fish. (And yes, they can get it on and make little ... gullies?) The only difference is that most mollies grow to be at least twice as large as most guppies, and mollies produce slightly fewer young.
Room and Board
Guppies and mollies are both mainly vegetarians who occasionally splurge on a seafood dinner. They should be fed plant-based flakes or frozen food with snacks of frozen or live bloodworm or brine shrimp. You can also treat them to fresh romaine lettuce or cucumber.
Both are brackish species -- they like their freshwater a little salty. Both thrive in heavily planted tanks with warm, slow-moving water. They mostly get along with each other, but not with more aggressive species.
A Rain of Fish
Guppies and mollies have similar lifestyles. Gentlemen of each species enjoy romancing more than one lady at a time, and their lady friends can use a single encounter to produce many future batches of baby fish. Both are livebearers, and both have the unsavory habit of eating any young that aren't very good at hiding.
Each type of fish is most famous for prolific breeding ability. You'll soon have hundreds of guppies or dozens of mollies, unless you have a population control plan in effect. If you can stomach it, the simplest answer is to do nothing and let nature take its course. Your population will probably still be ever-growing, but much more slowly than if you intend to save each baby from the voracious appetites of the adults. Otherwise you will need to keep only males (zero population growth), or have a separate tank for the baby fish (here's hoping your friends like fish, too).
If you decide to keep guppies, you can begin with a tank as small as 10 gallons for up to five fish, which usually grow to no more than an inch in length. If you prefer mollies, you'll have to accommodate their 2- to 5-inch adult size from the get-go with a larger tank, one gallon per inch of adult fish.
However, in a sense all of this talk of beginning tank minimums is moot -- you are committing to a growing population when you commit to either fish (at least, if you ever keep females), and even if they weren't so danged good at reproducing, size does matter when it comes to having healthy fish, and bigger is always better. If you keep both together, plan around the molly's larger size.