There is just not much in the free software world, particularly for Linux, that is good for music composition. Just the way it is. If you want to do it well, you need commercial tools, generally for Windows or Mac.
For what the original poster is looking for, I'd say have a look at Cakewalk Sonar X3. Sonar is real, real good at MIDI, knows how to deal with SoundFonts, has some built in synths that aren't too bad, and only runs $100 for the basic version. It's notation is not the best, but anything I can think of that is a reasonable step up is also quite a bit more money (like Cubase).
However depending on what the ultimate goal is, the DAW can end up being the cheap part of things. High quality samples cost a lot, and there are few freebies. Reason is to make good samples you need to hire good musicians, a good recording studio/hall, good engineers, and then spend a lot of time on it. Gotta make that money back somehow. So if you want realistic sounds, you can easily spend far more on samples than the DAW/sequencer. I own Sonar X3 Producer, which is $500, but I've spent more than that on a single sample set, and I have multiple sample sets.
Also if he thinks that programming a synthesizer is easy, he's got another thing coming. Making a competent synthesis engine that sounds good, is usable, etc, etc is not an easy task. Particularly since there are all sort of different kinds of synthesis one might wish to use, and each is implemented and controlled differently.
So, like the parent said: religious statement or actual work? If you just wanna play around in Linux with free solutions, then go to it. No need to ask on Slashdot, just try stuff out. Wikipedia has a list of OSS music software, to name just one place. If you are asking because you want something that doesn't suck and can do some real work, then you'll need to stick with Window or Mac and drop some money.
Like I said, I'd go for Sonar. There's a free trial, and the base version isn't that much and has good features and capabilities (it isn't crippled with regards to tracks and so on). You can always upgrade later.
Other reasonably priced options to look at are Reaper and FL Studio Fruity Edition.