maby im biassed but id get a 100, especially since your coming from a vtr, my remapped dci 100 (£270 to have it mapped) is VTS pace, 2nd gear and up. 5th gear on the motorway its quicker.
ive had my 172 at work this week due to the nicer weather and on the motorway find myself dropping to 4th and sometimes 3rd when going for a quick overtake to make it in before a junction, in the 100 you just sink the boot in 5th gear!
your better off asking the questions over on cliodci.net as the dci's arnt well liked over here for some reason.