I think the two things you need to consider...
1) How much money is in your pocket
2) Screen size
Although I don't own either one, I own a Powerbook G3/400 (1999), I have played with both models at the stores, and as a sudo graphic designer, I woud love to have the wide screen the Ti provides.
The ibook screen at 12" is too small... my 14" on my Powerbook screen is just perfect, meaning... any smaller and it would not be useful.
But, if money is an issue... I'd almost rather you pick up a used G3/400 or 500 off ebay... than be stuck with a 12" screen... assuming that it was going to be your primary screen. If you are going to have a external monitor, and only use the laptop screen for mobile adventures, then you might be okay.
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