I could get a MacBook or an iMac for about the same price but with obvious differences. For iPhone development which one would be best considering these main points.
Stationary iMac versus portable MacBook:
As an iPhone developer which one is more important taking into consideration performance. I am not as concerned with screen size.
I am looking for any other considerations from people who have used both or perfer one method. I want to gauge which one offers you the most productivity.
Will you be working on the road? If so, go with the laptop.
I'm not so sure you shouldn't be concerned with screen size. I often have the emulator up and I often find that I wish I had more screen space for various Xcode windows.
As far as performance is concerned, i am sure they will both behave the same. the question really is a personal one, which do you prefer to do your development on? if you are used to developing on a desktop with a full sized keyboard, shifting to a laptop might pose problems. if, on the other hand you are comfortable developing on a laptop, it really is your choice. do you like to be tethered or do you like to move your dev environment around (home to office, business travel, to vacation, etc)?
I'm not aware of any relevant difference beside the screen size and the portability factor which you pointed out yourself.
I'd consider buying a MacBook Pro instead of a MacBook, though, simply because of the backlit keyboard. Also, I'd go for the 15" model (ideally including both graphics cards, but that's just my subjective opinion). You'll need some screen space because of the simulator. It would be really useful while debugging.
The emulator is one thing but to be honest on my 15" MBP I sometimes struggle with screen space if i'm using the debugger etc and trying to cross reference things in separate files at the same time. I wouldn't even consider going for the 13", its nice, but its not an ideal working environment(my mum has one), screen and portability will always be a compromise but seriously 15" is the smallest realistic workspace before you start sacrificing productivity in a big way for xcode. In the current line up that advice forces you to think MBP in that case if your not going mobile the iMac will likely be a better choice due to getting a bit more grunt for a bit less money.
I would recommend a 15" Macbook Pro for the purpose of coding on the go as well as the portability for meeting with other developers on your team as well as going to conferences, coffee shops, etc. If you want some more screen space when you are in your office, you could always connect your Macbook Pro to an external display.
Performance wise, either would work. Even a mac mini makes a fine development platform.
I personally have a 15" macbook pro, and make heavy use of spaces with four screens - one for debugging, one for coding and one for IB work (the other is for mail and web). I find this works great and I don't even use external monitors anymore.
For my money, it's a no-brainer: 15" MacBook Pro + external monitor, keyboard, and mouse.
If you want to sit at your desk all day, go ahead, and you'll have the benefit of 2 screens (rather than one on an iMac). But if you're feeling confined at the desk (as I frequently am), unhook the MacBook and go code something up on your La-Z-Boy. Or at Starbucks. Or on the beach. Heck, after you sell a million apps, you can take the MacBook to Cancun and write some code there. That's my plan, anyway. :)
When it comes to testing on the emulator, a faster processor can actually be bad. It's too easy to look at the response time in the emulator and decide that it's "fast enough" when on the actual iPhone it is too slow. A faster processor exacerbates this problem.
Of course, the real answer is to do performance testing on the device, but the emulator is just SO convenient.