I got a 670 on the Math I Subject Test but a 750 on the Math II Subject Test
Should I send the Math I score to colleges where it is optional to send it or should I only just send the Math 2 score?
Only send the Math 2 score. Math 2 is a more respected exam to take and you have a much higher score. However, if the college requires two SAT II scores, than you might have to send both anyway.
If you have the option of score choice, just send Math 2; no college wants/is impressed by both math tests anyway.
There is no point to take both Math1 and Math2 subject tests. Even if you score well in both, they are considered one subject. If the school allow score choice, you may send any score you want and skip whatever you don’t. If the school does not allow score choice, you have to send everything. They will mostly look for the better scores or whatever is required anyway.