Well, I a Visor fan, but if the Palm truly is available for 100 bux with no obligations I'd have to recommend that. However, I wouldn't sign your life away... the website is quite vague.
If the M100 is $150 and the Visor (not Visor Solo) is $180, I would still recommend the Visor, since the Visor comes with Pocket Mirror, which allows proper synchronization with Outlook. I don't think the Palm products come with that, but maybe things have changed - you'll have to check that. Pocket Mirror is $40 I believe. I also prefer the Visor's USB cradle, which is compatible with Windows 98, Mac OS, and Windows 2000. I use a USB cradle at home with 98 and 2000. Much faster than serial (which I use with my NT box).
The Visor's 3.1H and OS 3.5 are both quite similar, but one must remember that BOTH have some compatibility problems with certain software. However, most current software is compatible with both.
The Visor's springboard slot is cool, but on a tight budget it's not one of the bigger considerations for most things. But one thing I consider ABSOLUTELY necessary is proper backup. The HotSync software does not do proper backups to the PC. You'll need a third party program for that. However, if you can't always sync to your computer (eg. on vacation), you'll need an alternative form of backup. The only way of doing this is with Handspring's $40 springboard backup module. Contrary to popular belief, all Palm and Visor products will crash from time to time (but much less so than Windows CE), especially if you install weird software. I use BOTH a software backup for my computer and a backup module.
I am making these recommendations based on your budget. However, if you can swing it, I would definitely recommend a Visor Deluxe or a Palm IIIxe. The extra memory is definitely welcome. A friend just bought IBM's clone of the Palm Vx, and at first he used up around 600 Kb. Then I introduced him to some software, including AvantGo, and within a week he was up to about 5 Mb or so. (My AvantGo channels alone take up around 2 Mb I think.)
See the Visor website below and my FAQ for more Visor information.
And since we're on AnandTech, here are my specs:
Visor Deluxe 8 Mb with USB and serial cradles, overclocked.

Backup module
ThinkOutside Stowaway keyboard (this is majorly cool)
(still waiting to buy Innogear sixpack, which includes modem)