I did MP 3rd grade with two kids this year and we loved it. We do different spelling and math, and were only in Prima Laina but did everything else. I do think their science is light, and we supplement. The astronomy is only one one day a week so you could add in more if you wanted to (what we did because I have science crazy kids). We actually did Sassafrass too this year as well as other things.
The Greek Myths is definitely not to be missed, it has been the highlight of my kids year. I have never done TOG so not sure how much work it would be to do both. However, to ease the load you could just read the myths and do the guide orally.
We love the Christian Studies, I think it is outstanding and love the built in memory work. We are Catholic and also add in saint, liturgical and catechism to our religious studies. We do the MP bible study lesson on day a week, and the rest on another day. We do our memory work daily.
Hope this helps, as I said I am not familiar with TOG but we love MP.