I'm putting in for DS next time around. I'm also a former SP.
I used to work with USSS occasionally, and I've talked to a couple DS guys. I get the impression that there are many more lateral opportunities in DS-intel, ct, investigations, psd stuff, physical security, tracking down American fugitives in foreign locales. USSS SAs concentrate more on psd and some investigation. The USSS guys also work their asses off on psd compared to DS. I remember being posted with one Secret Service agent, smoking hot chick on the POTUS detail, who'd worked the last 22 days straight, 12-16 hours a day. DS would, of course, involve far more overseas postings vs. shorter deployments for USSS.
just my .02. I'll restate that all of these are impressions and I would appreciate any other input or corrections.
Also, if you'll have 18 years of experience when you seperate I'm assuming you'll be 36. 37 is the cut-off for both agencies. Dunno if it's waverable. It ain't.