I'm in a Type VIIC now but previously in this career I'd done 16 patrols in Type IIs.
I think you'd be crazy not to upgrade to the Type IID. That extra range is SOOOOOO much better. More than three times the range of the Type IIA. In the IIa you can barely make it up to Scapa and back. In the IIDs you can escape the shallow and dangerous North Sea and comfortably cruise around the deeper and richer hunting grounds of the west coasts of Ireland and Scotland. The IID also has a slightly better submerged speed.
Also, if you're using GWX and in 1st Flotilla, the Type VIIs aren't going to become available for awhile so the small amount of renown needed for the IID is trivial.
Even if you were planning to upgrade to VIIs as soon as they are available (which it sounds like you're not) then it's IID FTW!

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