I used SL for a few years and loved all the books, but I tweaked it way too much. I'm using MP's full 4th grade with dd9 and she is thriving! We don't use R&S, but we use ALL the MP stuff.
Your dc are pretty young and I think doing all MP with them would be time consuming unless you can combine them in a few subjects. Tanya at MP is great at helping you combine dc! That being said, I'm not sure how long MP would take you so I'll just share what our experience is.
My dd does a lot of MP on her own. We do the first lesson of Latin together so I know she has a good grasp of it and I do daily reviews with her. She does all her own reading for Famous Men, Christian Studies, Lit, Science and Geography and we do most of the questions together orally and I do daily reviews of "facts to know" and recitations. She is usually done with all subjects by 2:00 so she has plenty of time for reading. I've got a lot of SL books that I assign her in the afternoon.
So, I think I would pick either SL or MP to be your main curriculum and just add in components of the other one. I think MP and SL complement each other pretty well!