You will indeed have to upgrade your T6 lvl1 eventually, but I would delay it until you have unlocked 17-19 HF lands (I even went for lc first...).
Statwise, adding a new T6 lvl1 gives you a lot more stats per gold until around land 17 (sorry, on holiday, I won't calculate it over again... catching some sun).
Plunderwise, adding a new building always should yield the most. This however will depend on your actual size and the target you're hitting.
Once you have T6 lvl1s all over, upgrade them to lvl2, and right afterwards to lvl3. Don't go and upgrade all of them to lvl2 first.