Well i have both and the sled and i really dont have a preference. I would not sell either to obtain the other. Actually this is not exactly true. I have the C3000 and i would sell it for the C3100. haha.
To be a bit more helpful, i use each for a bit different purpose. I like the 6000 better for regular PIM duties and day to day use. The C3000 is much better for typing (keyboard is much nicer) and web surfing b/c of default landscape orientation. I dont really like the 6000 in landscape w/o an external keyboard.
Concerning storage, you dont have the full 4gb drive for program storage. I dont know exactly what you get with the C3100 but i only have 400mb for storage on the C3000. The rest is for USB mass storage. There are topics that lead you through partioning the drive to be able to use however much you want for program storage but they require a bit of hacking.
Although i have the sled for the 6000, i hardly use it. When i first got it i used it all the time but i like the smaller size. If using the Guylhem rom and underclocking the battery life is sufficient. I also have no need for 2 CF slots at the moment. I will after i get a bluetooth card. I have just ordered a treo 650 and plan on using Blue tooth DUN to connect my Z to the net. Pending i can find a socket BT card? (anyone have one for sale?
).
I have not tried PdaXrom at all. I would like too but i have not found a nice step by step install guide for the 6000 yet. IF you know of one let me know. I am sure it is easy but i would like to read one first.
My opinion is just buy a C3100 and then decide for your self. Prices on Pricejapan are up right now but when they come down i will probably upgrade to a C3100. The only reason i would trade the C3000 for the C3100 is the availability of ROMs. Hope that helps.
-Mark