Hello, I am new to this forum, but have been reading posts for a couple of years. This post gave me the push to register as a user. When I read math posts, I want to agree with those who recommend MEP. We LOVE it. For me, it has the perfect balance of hands on, team work, individual work, logic, word problems, drill, and many of the things "mathmarm" mentioned above. I did year 1 for 1st grade. In the UK, where the online program was published, their year 1 is more like the US K year. In Hungary, where the program was developed, their year 1 is more like US 1st grade. My son learned his number facts from 1-20 backward and forward going through year 1. The spot we are at now in year 2 "seems" too easy because it is reviewing whole tens and 5s to 100. I haven't moved my son forward, because although he knows how to use whole 10s and 5s very well, he is learning to use them backwards, forwards, and up side down (if that's possible). Year 1a starts very easy, more early K. I started my ds further on around mastery of the number 5 in the lessons. Even though it deals with small numbers in year 1, like mentioned above, there is mastery of those numbers. I wholeheartedly agree with mathmarm that you don't need a program, but if you want a program and want to follow mathmarm's advice, MEP meets most of the criteria mentioned in the list.
I know you asked about 2 different programs, so I want to be careful not to "sale" you a 3rd program. If MEP wasn't free online except printing cost, I wouldn't mention it. From what I've seen of MathUSee, it should be fine to combine. I like that there are videos, manipulatives, and worksheets with MuS, so it may be good for your ones that learn differently. I haven't seen Math Mammoth, but think it has similarities to MEP. I did buy an expensive math program starting out, but resold it to use the free online program instead. If MEP wasn't available, I probably would have made my own program up like mentioned above.
Whatever decision you make will be fine. I like the advice I read before, to choose the method or program that suits your learning style as a teacher, because you are the one that has to be comfortable with it in order to teach the material well. Any program can be modified for a more visual, auditory, written, or hands on students.