Hi B-Man,
First off; I'm no experienced pilot (mere student with a paltry 10 hours, learning on Tomahawk). However, here will be the general differences:
As you state, the Tommy will be cheaper to learn on. PA-28 will be a lot more stable; PA-38 twitchier and less stable (not a bad thing when learning).
Don't believe all the bar-room bull**** that is regularly spouted about the Tomahawk; about the trimmer being supposedly "crap" and the "dodgy stall/spin characteristics". Rubbish. It's a fine aeroplane - as is the PA-28.
At the end of the day, it's up to you. What do you feel you would be better off learning in?
Like I say I'll recommend the Tommy because I'm learning on one and it is a nice aeroplane, but don't take my word, it's up to you at the end of the day!
Smithy.