Well I have both - before you ask how, I have the Tech Pack in my S3 and the SD Card version in the wife's A1, and have to say the Tech Pack is better. If nothing else it's a little quicker re-calculating routes, the screen as mentioned above is slightly larger, and I find it gives slightly earlier "advice" of route changes (the SD card version tells you OK, but sometimes you could be right on top of the junction). The quality of the displayed map is just slightly crisper as well.
That aside there are few differences, from a navigation aspect both work extremely well. But given the choice, then I do prefer the Tech Pack, and think if you use Sat Nav a lot (I do in my job), then its worth the extra outlay and I'm glad I have it in preference.