I think another thing to consider is what kind of work load are you expecting from your visual artist or programmer?
I'd imagine the ease of locating someone unpaid would become smaller as the scope increases. If it's some pixel art that can be knocked out in a few hours, or some specific assets like background artwork or character animation (for a small game); I can see some people contributing to add to their portfolio, or if the programming was a few helper algorithms or building a small system that helps the overall project, maybe even game logic for a small game (like a gamejam game), but if you were after one person to do all the visual assets for a large game, or wanted a single coder to produce underlying systems and game logic for a large game; I imagine those people would be hard to find.
Though this is only my opinion, maybe it's worth consideration (who knows);
I have some experience with programming & visual assets, and I hope to start contributing to other peoples projects (eventually working towards paid commissions), but out of the two, whether paid or free; I expect to stick to contributing visual assets and not offering coding services.
My reasoning is that if I'm offering visual services; it's quite easy to figure out what the client wants, whether they give examples or you can quickly put together some loose samples that can be used to try and figure out what they want. It's also not too hard to jump into a project that already seems to have an overall style and adapt to it; contributing small sections. It's also quite fun to be working on more than one project, switching up style and technique.
I consider programming somewhat different... Even if a person hasn't got a background in visual arts, they can tell you what they like and offer direction, but I'd say coding is a little different. If I was to do all the programming for a project; I'd need to have a clear understanding of what the complete project is intended to be, maybe even a game design document. Programming for a person who doesn't code is a process of turning abstract concepts into working logical systems, which is pretty much what you do when you direct your own code, but if they decide to make changes along the way, you might have to do massive rewrites, which sounds sort of annoying.
Another difference is that even though it may seem that having more coders will make the process easier; there's a high chance that it will cause additional complications... As it was said in "The Tao of Programming":
3.4
A manager went to the Master Programmer and showed him the requirements document for a new application.
The manager asked the Master: "How long will it take to design this system if I assign five programmers to it?"
"It will take one year," said the Master promptly.
"But we need this system immediately or even sooner! How long will it take if I assign ten programmers to it?"
The Master Programmer frowned. "In that case, it will take two years."
"And what if I assign a hundred programmers to it?"
The Master Programmer shrugged. "Then the design will never be completed," he said.
That book is intended to be a humorous text, but I think there's some truth to it. Though I'm really just guessing; I'd say that it would be easier to locate a visual artist, though a programmer might be more in demand, so I'd assume (depending on scope), a programmer would be more expensive, though in reality; I have no clue.