The Pentium M 1.7GHz CPU will be faster than a Pentium 4-M 2.6GHz CPU under many circumstances. You can't directly compare clock speeds. The Pentium-M is built on a completely different core designed from the ground up for mobile applications.
When plugged in, the 4-M will run at full speed and be slightly faster, but when running on the battery, the 4-M "SpeedStep" technology kicks in and slows the machine considerably. The Pentium M uses much less power and can run at full or nearly full speed even when in power saving mode. The Pentium M is far superior for truly mobile users.
The Mobile Pentium 4 (on the Inspiron 5150) is yet another processor that at first glance looks ideal for workstation users with occassional need to take SolidWorks on the road. It is simply a repackaging of the last generation desktop Pentium 4 with 533MHz FSB. The system architecture is much less desirable. It uses old fashioned SDRAM - not DDR and not dual channel - and this might completely cancel out the benefit of 3.06GHz clock speed. Battery life is dismal and the machine is heavy. On the upside, it's much less expensive than comparably equipped notebooks that are optimized for mobile use.
All in all, the M60 is hard to beat. You can even order it with a 7200rpm hard drive. It's slower than top-speed desktops but faster than ANYTHING that you could by just a year ago.