I use both, project dependant, but in my personal opinion DVD SP has a far steeper learning curve and could do with a bit of a re-jig in terms of layout. Encore is quick to get to grips with and you can round trip with Premiere easier than the FCP>DVDSP.
As mentioned before, if you're using Final Cut Studio then use DVD Studio Pro, otherwise, if you don't own a suite yet, it might be worth looking at the latest Adobe Suite as it has some really great tricks to the package and you get great software like After Effects with it. Personally i prefer FCS2 but hey ho!
Also bear in mind that it it reasonably likely that both suites will recieve major upgrades in the next 18 months (x64 bit Adobe CS5 as well as, hopefully, Final Cut Studio 3 optimised for Snow Leopard).