I'm a newcomer to the franchise so I was wondering if it is worth getting Heroes 5 if I own Heroes 6?

Definetely, buy it. You will get Collector's Edition of the original game, as well as both expansions (Hammers of Fate and Tribes of the East). It does not have any DRM or any stupid online-only features (actually for this reason I never bought Heroes 6).

I actually grabbed The Heroes Pack from Steam since it was on a sale recently I was able to get it pretty cheap and it includes both expansions along with Dark Messiah.

if you got a really good computer Heroes V is a must
it's basicly the most improved Heroes Of Might & Magic game out there

the campaign has a fantastic story and will suck you in the world of ashan

the multiplayer side is pretty awesome

you either go the fully engaged battle with a setup heroes and items or do it in the old traditional way of a Scenario

and don't bother with heroes 6
utter piece of s***

great story but wonky combat system and dogs**t magic system
also heroes 6 is bundled with ubisoft's nazi DRM with takes forever to load

so in other words
YES buy heroes 5
BUT avoid 6 (watch the cutscenes on youtube cause the story is pretty beast)

The best thing about HoMM V is the skills system IMO.
The story is rather weak, and the voice acting is cringeworthy.
The 3D doesn't add anything worthwhile compared to HoMM 3 either.
Still, definitely worth it.

Definitly. I prefer it way over H6. For the reasons the posters before me have mentioned plus...

...there is a AI patch (by a professional programmer and allowed by Ubi) in development and you can even get a stable beta version of it, which already improves the games AI significantly:

http://forum.eternal-essence.com/