I gotta say right off the bat I really know very little about this. But from what I understand, Pentium M is the processor specification, I believe it has twice the cache of a P4 (which makes it faster at the same frequency) and it offers frequency scaling for power saving. What this means is that when you're playing CM your cpu runs at full speed because you need all the power you can get but when you're writing a letter to grandma in notepad, the frequency will drop from 1.6GHz to say 600MHz, save you battery power, and prevent noise from the cooling fan which isn't needed at those speeds.
I got a Pentium M 1.6 and I'm quite satisfied, never run it faster than 800MHz (which is the breakpoint where the fan starts up cause it needs cooling) when I'm not compiling anything. Can watch movies at 600MHz just fine for instance, handy for long train rides and such, have about 3h of battery time then.
Now, the Centrino keyword I think is just a buzzword from Intel which describes an integrated mobile device system. I think it includes the cpu, however it also includes an Intel chipset and I assume a wlan card (which I have).
The second one.. does it come with a spare battery? Because 10h sounds downright unrealistic to me, in fact the battery specs are always greatly exaggerated, my battery is supposed to be good for 5h according to the marketing wording, it's actually 3h. So 2h will probably give you around 1h on the first model, which is very little. The first one has a definite edge on the second but the question is how much power do you need from a laptop, the second is still very decent and much lighter too.
Yeah they say 12h with additional battery, means you have to switch when the first one runs out. Still don't expect 6h from each battery, more like 3h+ is more realistic, I don't know how they test these batteries but I have never heard anyone actually get the time they advertise.
Apparently choosing a laptop manufacturer is a very personal experience. You'll read a lot of horror stories about certain companies if you look around for opinions. I've heard a lot of success stories about IBM notebooks. I have a HP/Compaq myself (Compaq production, HP branding basically) and I'm very satisfied so far. As for Dell, it seems that either you'll be very satisfied or you'll have serious problems.