RE: Number of keys... well this depends on you really. If you want to play it like a piano / keyboard, then the more keys (octaves available), the easier it is to play methinks. But, if you're not really a keyboard player it may not be so important. However, if you want a very portable setup, then the smaller models will suit you well.
By dedicated controller, i meant a "Control surface". This would be a piece of hardware with only knobs, faders, etc and no keys. The idea being to act in a similar way to the mixing desk you see in pro studios.
The benefit of having lots of knobs, faders, pads and buttons is that in Logic you can assign each pf these to a different function!
Instead of having to remember that key command or menu function for... assign it to a control on your Controller!
Being able to add automation to your project, e.g. adjusting volume, pan, EQ, + whatever else(!) by twiddling knobs on your controller can be a lot more intuitive and fun than using a mouse to adjust one parameter at a time...
The axiom series does look very good.
Choose the one which fits your most important needs: e.g. mobility, price, number of controls or number of keys...
They will all be suitable for beginners. MIDI is not such a scary world anymore. It;s very accessible for all, especially in Logic 8.
By the way, in reference to the tip you were talking about, Steve H. walks through this feature (called Pattern Mode) in video 25 of the Ultrabeat Unleashed tutorial (if you already have it - you may be interested to check it out :)
p.s. i use an M-audio Oxygen 8 & personally (for my needs) if i had to upgrade, I'd go for axiom 49 due to it's faders, extra octaves etc... :-)