You want to know the levels of the brewing ions in the water.
RO water will be low. Spring water will be whatever the spring files through, it can be low ions or high and be called spring water.
This^^^
I would use RO..... For starters, it is cheaper if you go this route and just refill from those machines at grocery store or walmart (.39 cents usually). "Spring" water can be double or triple that.
And, as pointed out - you really have no idea what "spring water" is. RO water is basically a blank slate..... so you know what you are starting with.
Until you know exactly what is in your water, you are basically flying blind with trying to do much with it. That said, carbon filter to get rid of chlorine. And if you went 50/50 RO water with your water, there is a good chance you will be in the ball park of fine for most any style. But, ultimately, you need a water report and you need some sort of software like Brunwater to really know what you are doing (or just go 100% RO water and build back from there).
It is worth it though - my beer improved dramatically when I began to understand some basics about water. I would not consider myself an expert, but I have a working/functional knowledge of what I need to do. I brewed mediocre to bad IPA's and APA's for 12-15 years all because of my water. As soon as I figured out my water I began brewing good-great IPA's.