Also using RS math with boys who just turned 5 & 6. I too wanted "something workbooky," if anything just to see on paper that he was "getting it" without me asking oral questions or showing me with manipulatives. I want to see them do some traditional math work too.
In early november, I ordered singapore earlybird for my 5yo and horizons k for my 6yo. So far with singapore, I wish I would have ordered the textbook instead of the workbook. I can see the difference in the way math is taught from RS math. Singapore teaches something, then goes on to the next concept. Now for Horizons, my 6yo is flying throught the first book. It really is just reinforcing what we have been learning and showing me "hey, he knows this, and this, and this." Horizons is more spiral and doesn't challenge but reinforces right now, since we are furthur in the year with RS.
I am still courious about Math Mammoth, and it is quite inexpensive, especially the electronic version and I have heard others say they liked how it lined up with RS. Another consideration someone touched on above is going through a workbook that lines up with your state or national standards so know where you stand on them with your kids.
Hope this helps. I am pretty new to homeschool, but have done so much research, (too much maybe?) and feel like my choices are overwhelming. I am coming around to realizing the reality that we have only so much time in the day. Lord help me become better!