Sell all your inferior necas and buy both!
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..but seriously the VI will possibly be delayed like IV and Warmachine so it may be best to get the IV when its released and when the VI finaly comes out you will see how the articulation is and whether it is the same as the iv ..and then you can sell the iv and get the vi if you want ..or just buy it and have both if you have the money saved up.
O and those proto pics of IV hitting the ground ..ya the articulation is obviously an improvment over Mark III's but i'd still take the poses in those pics with a grain of salt, we won't know what the articulation is truley capable of until we have the fig in-hand.