Morbyd-im not sure how much experience you have with virtual instruments...but if you're a seasoned songwriter, like u said, who wants to quickly create melodies and rhythms with good arrangements with ease, I suggest Reason 3.0.....Cubase is more like a Pro Tools where midi programming isn't the most convenient (though i heard Protools 7.0 will be ideal for midi programmers), these two softwares are more for recording, mixing, and the mastering stages. I started producing 4 years ago and didn't know how to play lick of anything...so you're pretty much ahead of the game..
Reason is ideal for easy midi programming and a user friendly interface..It has a kool REX loop player, two different samplers(with filters, envelopes,lfo's,etc.), two synths (with as much features as the samplers but twice as more), and madd samples and also and Orkester sound bank with some great sounding string, brass, and woodwind instruments. Also their drum machine allows a user to load up any type of sample you want in each of its channels..Sequencing is a breeze also-editing notes and/or bars is simple & easy, ideal for fast & uncomplicated editing...Mixing on Reason is a biatch though, cuz you can't mix in real time and u freakin have to write in all your automations...instead u can just dump all your tracks on the Cubase you got and mix there..