Both have their pros and cons. I used to order around 6 months worth at a time from online sources. But sending multiple money orders and dealing with fresh addresses and deliveries was such a headache. I'd normally pay close to nothing because of how tight I'd get with sources though, and when you have good rapport you'll find extra vials and samples of orals in there all the time. Build a good relationship with your source whether it's in person or online.
I normally don't buy things in person because the selection is usually a bit more limited, and because they've already gone through the trouble of brewing or acquiring their stash you can usually expect a mark up. But if you find a good, honest source in real life sometimes it's worth the extra expense. Sometimes you might just want an extra vial or two and putting in an online order is out of the question. I thankfully have a great local source who always throws me extra vials even if I'm just grabbing one or two items. When my daughter was born he said she was so cute that I had to get bigger and uglier to scare away the boys, so he gave me 6 vials of test and 6 vials of tren.
Always make sure to be a positive and delightful customer. Even if you get poor gear, approach the issue professionally and don't be a fucking whiner. Juicers are the worst recreational drug users in the world. No one's a bigger pussy than a juice pig who's crying because they suspect their gear is bunk. Sometimes there are problems, but sometimes juicers exaggerate and are big drama queens. I've found that as long as you approach and business problems with courtesy and respect, you won't have any problems. I've had a few shitty vials and because I approached it professionally, I was always reimbursed. I usually get an extra few vials for not being a douche about it. Some sources really do care about the quality of their gear, their reputation, and keeping their customers happy. The reason why I've never had huge issues is because if I feel a source is sketchy, I'll simply take my business elsewhere. If someone's a sketch, don't stick around with your business. Pack it up and go to someone else who's going to be more serious whether it's 2 vials or 20 vials.