my two cents.......
i have run Photoshop, Illustrator & Quark on my G3/333 lappy which i bought new in 1999.
while i won't claim it's the fastest beast on the market (it was when i bought it tho'), it has happily served me very well, not to mention been the envy of PC using colleagues right up until today (actually, yesterday when it whooped a P4 lappy while scanning pix with a Nikon CoolScan II)
oh yeah, i only have 198 Mb of RAM, with RAM, the more the merrier.
i wouldn't concern myself with a .13Ghz difference in processor speed, you prolly won't notice it in "real life" unless you ran the two side by side.
on the battery-life issue - all claimed specs for this have to be taken with a very large grain of salt. perhaps you could reach Apples claimed max batt life - if you only ever use SimpleText, scale back processor speed etc (amazing how much of a diff that makes BTW).
my only advice is get the best you can afford and then fill the bastard with RAM, the diff between PB's and iBooks is pretty marginal, if you don't NEED the extra functionality in a PB, get an iBook and spend the cash you saved on more RAM/apps/buying drinks for that cute blonde at the end of the bar et al.