I have been both SPG Plat and HH Diamond for about 10 years. I value SPG Plat more than HH Diamond. But I do need Hilton for a lot of my business travels, so it's useful to keep HH around. When I wasn't sure about being able to maintain both top status, I made sure I requalify for SPG Plat, and try to maintain HH Gold or better to keep the free breakfast benefits.
If you are interested in SPG, then you should continue with Hilton and achieve Diamond. Once you're a HH Diamond, then ask SPG for status match to Plat. I did SPG Plat the hard way, 25 stays in the first year with no real benefits.