MPEG4 may compress better for the same quality, most experts estimate it can compress to half the file size, but this is a highly subjective measurement.
Each compression method has a near infinite range of parameter settings. The default settings used in different software packages can skew the results rather badly. It's entirely possible for a well set MPEG2 compression session to result in a better, smaller file than a poorly set MPEG4 session.
The professionals in the movie industry actually manually change the MPEG2 settings for different parts of the movie to maximize the quality while still fitting the results into the DVD size limits. There are some compressor implementations that try to automatically sense these opportunities for size savings (VBR - Variable Bit Rate), but their success is rather spotty. TMPGenc is one of the better ones.
All MPEG4 compressors I've seen use just one set of parameter values for the entire movie, though it's entirely possible for a user to go back and recompress the bad bits then edit and merge the best segments of both. So far, I haven't heard of anyone doing this.
I believe if they did, they could easily duplicate the DVD MPEG2 quality in half the file size. But if they rely on the default settings it becomes a matter of how difficult to compress the source material is and the excellence of their two pass VBR capabilities.
I'm not impressed with QuickTime's performance, Microsoft's WM9 codecs can do a much better job. Nero's REcode MP4 compression is pretty good at fitting a full length DVD onto a CD, but they cheat, resizing the video screen to something a bit smaller than a DVD's 720x480 (NTSC) or 720x576 (PAL). The diffrence isn't apparent on a TV screen though. Only on a computer monitor.
So the final answer? Your mileage may vary.
Be sure you have lots of time to experiment.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70