I haven't used speedgrade much, but resolve has been an industry standard for years. It's high-end, very powerful but also very complex - there's a decent learning curve to it, and it really requires a solid understanding of the grading process and underlying color theory to make the most of the tools it has. Speedgrade is not quite as complex or powerful, but is geared more towards getting results quickly - hence the 'speed'. So for the casual user it's probably going to be easier to get the results you want out of speedgrade, whereas someone who wants to get into the depths of color grading is going to want to put in the time to learn resolve. Resolve also has a clear career path if you're interested in becoming a professional colorist, and it really shines at the high-end when combined with a hardware control surface - that's the $30k version, although you can combine the lite version with smaller third-party controllers in the $1-2,000 range for a very powerful entry-level colorist's station.
That said, both have the same basic tools, and if you're mostly looking to do primary color correction (i.e. not a lot of masking and layering of secondary corrections) you can likely get similar results from either.