There is not much difference now between Eclipse + Spring-IDE and STS. The main difference is that when you download STS, you are guaranteed that all the bits and pieces work together. When you install Eclipse + Spring-IDE, you may have some compatibility problems and trouble installing or upgrading.
My advice is this: if you already have an Eclipse that has custom plugins installed, then go with Eclipse + Spring-IDE and if you are starting from scratch, go with STS.