We ran TT5 on PDA - very good but slow to pick up satellites on Costa del Sol, and Portugal was just a green lump.
So, bought Garmin but didn't particularly like it. It went belly up in the middle of Spain - software problem after 3 months, so we replaced it with TT740.
Much more to our liking, but not used abroad yet. Totally self contained, apart from power (PDA & Garmin both had trailing wires for arial / traffic updates), neat, quick, good screen, good volume, has computer voices which read street names and give precise directions ("turn onto A47 signposted Norwich") although some of the statements are incorrect (Kings Lynn signposted, not Norwich for example). We miss John Cleese, who we had on the old TT, but we could buy him; unfortunately you then only get "turn left".
Instructions for getting into the correct lane at busy junctions only work on some junctions and is not always correct - I had a nasty argument (non-violent) with a HGV in SE London in heavy rain, whilst trying to get to Abbey Woods because TT said I needed to be in lane 3, whereas I could have stayed in L2 and still turned right! However, these are minor faults (in UK at least).
It sometimes tells me there is a quicker route, using "live" traffic management (£8ppm?) but doesn't let me know where it goes, as far as I can see, because I'm concentrating on the road ahead - I like my scenic routes, planned with care, so tend to avoid his suggestions.
Battery life whilst off charge seems short, and it is often very low after a couple of weeks in its case.
You pays your money and you takes your chance - good luck.
Gordon