Scott,
All modern aircraft are equally 'good' at capturing and maintaining the ILS.
The difference you observed between a 737 and a 320 can be down to a number of reasons:[list=1][*]The intercept heading given[*]Finger trouble by the pilots in not arming the approach mode[*]ATC requirement to increase spacing and make the A/C intercept from the other side[/list=1]No doubt there are more reasons, but I 'no' no others at the moment.
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There are 10 kinds of people. Those who understand binary and those who don't.