As with any of these things, the only way to be sure is to carry out tests yourself. For example, the Panasonic DMR-ES10 is supposed to have excellent built-in filters for stabilising wobbly VHS images in pass-through mode, however from my tests I found it blurred high frequency detail coming into its composite input.

This page seems to have some comprehensive advice on DVD recorders. The general consensus is that later models (2007 onwards) tended to drop features, such as built-in tuners, additional recording modes and filters to improve the video signal, in order to reduce costs.