Too me, the MXT is better balanced and didn't feel as heavy as the Explorer did. I'm not sure what the listed weight for each is, but I would imagine there isn't that much difference with the advantage going to the MXT, weight wise.
Depending on conditions, the larger coil should get better depth on the larger pieces of gold while the smaller coil should do better on the real tiny pieces. As I'm sure you know, depth depends on a lot of factors, including operator skill with a particular machine/coil combination.
You might pose this question on the prospecting forum. There are some very knowledgeable people over there and I'm sure you'll get some good response from them.
Hope this helps,
Otto...