Good question. I did a one day trip a few years back and did get the hopper and I found it useful, I was able to start in WDS, do the few attractions there I wanted to do and then spend the rest of the day in Parc Disneyland.
I have to ask, have you been before? If you have, then its handy to know just what attractions you want to do in both parks and can roughly plan a way around to maximize your visit.
If you've not been before, well then it sort of comes to personal preference and what attractions you prefer. Personally speaking, and if I'm brutally honest I'd say that you're best in Parc Disneyland and to skip WDS for now.
Second question - you raised the issue of cost. Now this is dependent on your budget (and size of party) really.
Surprisingly its only €10 more for a 1-day hopper over the 1-day-1-park ticket. Not too bad, even if you only plan to do three or four attractions in WDS.
But if you don't think you will want to do some of the big ticket attractions (like me, I don't do Tower or RnRc) then you might as well save the extra €10.
I hope some of that helps.
"We're not trying to entertain the critics ... I'll take my chances with the public." - Walt Disney