Inokatsu and Western Arms M4 GBB's use the same internal system. The Inokatsu M4's have the edge on external quaility, however, and that's why they're retailing more expensive. I don't know of any parts that can't be swapped between them. The G&P ones are just WA M4's with the G&P aftermarket parts installed.
A 'better' gun is hard to nail down. If cost-efficiency is a higher priority than realism, then maybe a WE GBB is the way to go. If you want to invest time and money to make it perfect, or maybe realism is a huge priority for you, then you'd want to start off with an Ino / WA.
They all have blowback and they're all wicked fun to use. They all require more money and care than an average AEG. The rest pretty much comes out in the wash.
Look at your budget, look at the kinds of aftermarket parts available to you, read some of the failures / successes other owners have run into, and spend a lot of time on GasGunsDotInfo. I wouldn't discourage anyone from getting any particular brand of GBB.
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Nabisco Lobstrosity
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