In short.. Get Blender! Since you're a beginner it's not worth pouring k's into a software you have no idea how to fully utilize.
Well, simplicity is a relative term. For one Blender will be simple, for the other guy Maya. But in the end it depends on how much you use the tool. For example, if you start with Blender, as time passes you'll get better with it, developing it into a habit, and therefore, widening the margin to transition into Maya freely.
It's the format that matters
Almost any 3D software will do, the only difference is in how you export your model and whether Unity3D supports it.
For sure, the de-facto standard for some time has been Autodesk's FBX, hence most developers use Autodesk software. And, yes, Unity supports it, plus it's recommended to use FBX.
Then there is COLLADA. Rumors say that Unity3D doesn't have a full support for COLLADA.
But, yes, Unity has wide support for importing not so universal formats- *.3ds, *.max to name a few.
Haven't been working with Unity lately and haven't used other software than 3DS Max, therefore I cannot precisely say what formats Unity supports. But I've tried *.3ds and *.max- more or less, but they work.
Formats like COLLADA and FBX are meant to be used by default.
Developers have put a lot of effort in these formats, meaning, that
they will not only export/import with mesh, but with animations, that
you can bake in and all the other stuff the actual model will posses.
Plus, they are software independent. You can work with Blender, your
friend with 3DS Max and an outsourced modeler with Maya or XSI and FBX
will provide almost 100% support for every tool.
It's strongly encouraged to use these universal formats.
As for the software itself, everything will be OK as long as they support FBX and/or COLLADA.
I'd recommend one of these:
- 3DS Max
- Maya (from what I've heard, Maya is more complete than Max, but then again, it's more complex than Max)
Since Blender is Open-Source it's free for everybody, everything and forever. It has huge community, lots of plugins and so on and so forth. In short, a free, packed 3D software for almost everything you can imagine.
Autodesk products on the other hand are better developed, they're commercial products, therefore higher quality product, plus, relatively better community- resulting in more plugins and 3rd party tools.
Autodesk software is not cheap tho', but provides educational license for every single tool they have there for an exchange of stripped down license.
One more bonus for Autodesk would be the infinte amount of quality and really helpful plugins/3rd party software. But the plugins tend to not be cheap either.
As rojcyk mentioned, there are tools such as ZBrush (Sculptris is the stripped down, free version of ZBrush). These are mostly meant to be used for details, like, when you need to add that quality 6 pack to your Rambo guy.
There is a bonus for this, you only have to make a reference model, and ZBrush will allow you to add that extra detail much faster and more precise than the plain ol' modelling tool, hence they fell into the category of "Sculpting tools".
It's not the tools it's the carpenter
The information above should be enough, but yes, modelling as for everything else in this world, doesn't just "happen". Takes a lot of time and a lot of knowledge to end up in something really usable.
Good luck and have fun!