Personally, I always start with the IX Boats but if your saving up for one I would suggest waiting for the IXC.
The IXC has better range/upgrades and becomes available shortly after the IXB.
Be prepared to totally change the way you hunt, these IX boats aren't everyone's cup of tea but if you find the correct way to use them then they are very rewarding.
I don't know if it's modelled in SH3, but as they are bigger boats, they are a larger sonar target and as such you will have SO much fun trying to evade escorts than you would in a VII.
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