Hmm, I doubt well water will be any better. City water is going to be treated, so there's some chemicals in there. But there are going to be a lot of minerals in well water, plus other "unknowns". I have a feeling that the minerals and such might actually be better food for the algae compared to city water, but that is just speculation on my part...
A TDS meter would help a little in determining which might be better, but would still leave a lot of unanswered questions as to what is actually in the water making up the Total Dissolved Solids...
The point of using RO/DI water is to start with completely pure water (0 TDS or as close as possible), thus limiting the amount of "unknowns" into your aquarium. In many many cases, the "unknowns" are responsible for unwanted algae growth and possibly worse, such as problems keeping fish and/or corals alive.
I went maybe 2 years on my first tank using tap water because I couldn't justify the cost of RO/DI setup (didn't want to waste my money on RO/DI when I could be spending it on cool corals
. During that time I battled algae problems constantly. This might sound dumb, or obvious, but it wasn't until I realized that the water is like 99% of what I put into the aquarium that it finally made sense to me to invest in RO/DI.
I guess what I'm trying to say is you can try a lot of things to battle algae, but in the end if you can cut off its food supply then the battle will be won! First place to start in that regards is the water...
HTH,
Rob