Technically, the Class 450s achieved 47598 Miles per Technical Incident (MTIN) for Period 7 2012 - 2013, whereas the Class 377s achieved 29761 MTIN over the same period.
From my view, both the 377s and 450s are on a par with each other: they both have their downsides in certain areas, e.g. rock hard sets or cramped interior design, but I felt the 377 had more of an ambience than a 450.
From a suburban railway point of view, a 450 is better because there are more seats than on a 377.