Hi Michael.
If your new to both notation packages, if it was my choice i'd stick with Sibelius and upgrade to Ultimate.
VSL have worked wonders in my opinion with there sound sets and there libraries, and everything works wonderfully well. So well in fact that i really dont use logic pro anymore, and do all my writing in Sibelius with VSL's libraries.
As far as i can tell, for every library VSL bring out, they also work on a sound set so it works in Sibelius (and they are currently working on a new one for the Special Editions 5, so if you've bought those they'll work as well)
I've toyed with changing to Dorico myself and have tried the demos availabe, but to me, its not as accomplished as Sibelius and as everything works with VSL i'm sticking firmly with Sibelius.
As a side note, as VSL works with Sibelius, so to do there other software, VE PRO7, VI PRO etc, so your mockups can sound exceptionally good, so you get the wonderful engraving of Sibelius, and the killer sounds of VSL...perfect really!
Sibelius ultimate has been updated a lot lately, so if your considering getting it, you may want to subscribe to it as i do, rather than purchase it...but that is of course completley up to you!
Good luck mate,
Kindest regards
Paul