Buy the AA-520.If you cannot afford it,buy the AA-230.Why buy the MFJ when the Rig Expert is within your budget?
I agree - as the owner of two analysers an MFJ269 and a VK5JST kit built , I have found they are both rugged and
efficient as far as my experience goes.
But - times move on, and the neat display and extra features of the RigExpert units would make them nice to use.
As others have said, you can figure out almost any attribute of an antenna or feedline from just the resistance and
reactance of the antenna reading, but if the device does the calculations, why not go with it.
Even though I don't often use 70cm - I would go with this functionality, you may need it and then you will be glad
you have it. $100 is not much more for the increased functionality. As regards different feedline impedances - not
very important as its pretty easy to calculate this with simple math, 50 ohms will be your most used anyway.
Good luck and have fun - an antenna analyser is really a Digital voltmeter for RF and you will not regret getting one.