From what I understand, mechanical keyboards(with mechanical switches for each key) are popular among professionals with spend a lot of time with a keyboard.

With better accuracy comes much higher WPM, words per minute is a count of words typed correctly in a minute

I personally use a mechanical keyboard (Brown Cherry Switches) and I must say I wouldn't want a regular one ever again. It takes some getting used to, but the feedback from the keys is much better and typing for extended amounts of time is less strenuous.

I dislike notebook keyboards and noone who can touch-type would even consider using an Ipad or anything without actual keys.

I'm at about 90 WPM average now, but I'm well above 80 with a normal keyboard, too. If you struggle to reach 35-60 you probably aren't touch typing yet. Learn it! It took me about 2 weeks (from being a hunt-and-pecker for over 10 years) to get to a 50 wpm average.

On the topic WPM vs accuary... It is difficult to improve your speed without trying to be accurate. It's easy to get sloppy when you want to be fast, and your speed will suffer accordingly despite your frantic attempts. Been there, done that. The best way to improve your speed is to try being as accurate as possible, and let tons of practice handle the speed issue.

Yes, it can make a distinct difference. I use a couple different mechanical keyboards: One with Cherry MX Blue switches, and a Topre keyboard that has a special kind of rubber dome switch joined to a spring, that uses capacitance (rather than continuity) to detect a key press. I prefer the Topre of the two, but both types are excellent.

When I compare either of the above keyboards to the built-in keyboard on my Macbook Pro, my speed and accuracy slow down on the Macbook.

I too use a Macbook Pro, but I have my own wireless keyboard that has larger and more elevated keys. For beginners like me, being able to tell what key your finger is over is important. Sometimes using my macbook, I would accidentally pass my finger over a key because I didn't feel it. But once you've got down hitting the right keys and are getting barely any mistakes, once you've reached the point where you only need to start going faster and not just better, then small keys are swifter and quicker to type with.

"once you've reached the point where you only need to start going faster and not just better"

Thats BS. You'll not get a better typist by faster muscles, but by faster keypresses due to ergonomical movements. You'll always need to get better to get faster. Also, world-class typists use mechanical keyboards, too. If they'd be better on a flat mac-book keyboard, they'd use one.

I really prefer using mechanical boards with blue switches... but outside of that the big issue for me is the size/scale of the keyboard.

I prefer tenkeyless for number and letter combinations because I don't have to move from the homerow area in order to get numbers. Also, there is less movement from the keyboard to the mouse if you are right handed.

All in all... size depends on your style, but mechanical is the way to go.