You might get a slightly better price, but:
You'll have to do some assembly yourself.
You won't make friends with your local bike shop, which is where you might need to go for repairs.
You can't test the fit to your body size.
I think from choice I would prefer to buy in person myself.
You don't mention what your budget is. If you're thinking of something sub-£100 then good luck but don't expect it to work that well. Some do, some don't. They're certainly horrible to work on - I've helped people with three of them recently.