Lots of good suggestions here, and many items to demo and consider.
I think the top 3 contenders at this point for my intended use are (these all run $99 for first timers):
Tracktion Waveform Basic
Image Line FL Studio 12 Fruity
Steinberg Cubase Elements 9
Waveform has some cool new MIDI composition tools now. You can set a project key, and then when you add a MIDI clip, you can choose the type: chords, bassline, melody, or arpeggio. Then the Pattern Generator will work with you to suggest chord progressions in the PRV to work with the scale you selected. The melody assistant is cool because it shows the ghost notes on the PRV for your chords in the other tracks, so you can drop a melodic line over any of them and remain in key!
Waveform is not going to write your song for you, but like a good spell checker does for a word processor, it will help you to spell correctly!
FL Studio has some useful under the hood MIDI features that I was not aware of. Besides having one of the best Piano Roll Views I have ever used, it offers ghost notes for your other tracks, so as you layer tracks you can follow your chord and scale pattern. It doesn't suggest progressions, but it does offer a framework for you to glue your MIDI tracks all together in key. The program doesn't seem to have any concept of a project key, so that part is all up to you.
Another cool feature with FL Studio is the stamp tool. This lets you drop a chord shape into the PRV, like a Major 7th, or a scale type, like a major or minor, and it will then let you drop them as a group of notes that you can move and arrange together.
If you drop a Major 7th, you will get the 4 notes in the relative intervals for that shape, so that you can move the group up and down to any root note. Same for the scales, except that you get every note in the scale. Move it to the root note you want and bingo! It also has a few other tools.
And lastly Cubase Elements. This cut down version has the Arranger track, the Chord track, Chord Assistant with Circle of Fifths. This is all very cool stuff, but it comes with the learning curve of Cubase. I must say, that as far as other DAWs besides Sonar, I think the UI designs of Cubase and Studio One are the best looking.
Decisions, decisions ...
