While you are doing the shocks it is time to replace the front and rear sway bar bushings. Easy job to do, the bushings a not expensive and results in flatter more comfortable cornering.
As far as Comfort vs. HDs, it depends on what you want. I don't see a big difference in the two. Yes, the HDs are a little bit firmer, but not dramatically. The Bilstein Sport shocks will make that difference.
As far as just the rears or both? I would do them all together at the same time. I have done rears only and that made me immediately do the fronts. Shock replacement is, in my opinion, one of those gratifying car repairs. You spend time and money and the improvement is something you can enjoy for years and miles into the future.
I recommend: Do all four shocks with HDs and replace the Ft and R sway bar bushings. Your car will feel great afterwards.
Keep us posted,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
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'65 911 Coupe (#302580)