I've been sewing for about 15 years. Sewn hems are very fast to produce, really strong, and very inexpensive compared to adhesives (I mean, we're only talking about the cost of thread). With sewing you can add webbing, D and O rings, reinforcements, etc... things a welder can't do. My favorite part is the appearance. It just looks better, more handcrafted. That being said, the relatively smooth / flat finish of a welder looks really good on certain banners, especially indoor ones.
Yes, you'll need a proper table and guides and such for an efficient workflow (and patience!). Hopefully you have the space for it. I built a custom air hockey table for mine that's 6' x 24' long, using melamine panels for the top (drilling approx 8k 1/16" holes) and powering it with an old HVAC blower motor. It was a lot of work to build, but I'm happy with it. (I wish it was bigger, but didn't have the room)
Sewing machines can be finicky if they aren't tuned / maintained properly, and you'll be lucky to find anyone local that can service them when they break down since industrial sewing machines are considerably different from residential models. I highly recommend researching these machines
and their maintenance routines to figure out which one(s) are the easiest to take care of. For the most part you can just keep everything clean and oiled, but you really need to know
where to oil everything. Just don't overcomplicate your machine with more features than you
actually need. A new machine with a good set of modern features can be had for about 2.5-3k (depending on how you want it set up).
Compare that to welders which cost... what... like 4x that much? No thanks. 
When buying a sewing machine, make sure you're not buying a Chinese model. Aim for the Japanese ones (Juki, Consew, etc). Of course you can also go with an older machine to save bucks if you don't want modern features (it's a good idea to have a backup machine anyway in case you want to start off slow and buy a nicer machine later). The older Singers (35+ years old) were built like tanks (before Singer sold their plans to China). I've got several of them, and a really nice Mitsubishi from the 80s. The one I use the most is my new Consew. I use it the most because it has the largest bobbins I could find (U bobbins). The larger the bobbin the better.
