I like Kelly's idea of doing K with both students if you can only afford one program. There is a thread here with folks who have done that, even after doing another K program: http://board.mfwbooks.com/viewtopic.php?f=1&t=13578
K is designed to be an intro to school, or in your case an intro to homeschool. However, it also has some solid skill-building so that kids really get the idea of phonics, rather than just memorizing words as they are so very good at doing. In addition, it has solid character-building, which will help them throughout their school years and beyond. Plus there are activities, literature, science topics, and more.
I started "afterschooling" my grandson with the K program when he was in kindergarten (and he was an older kindergartener). We didn't finish, so we continued using MFW-K while he was in public school 1st grade. You might want to cover up the grade label, because as a public schooler my grandson paid attention to that, but after he adjusted to the homeschool mentality that it's all good stuff to learn, he enjoyed the lessons and we just started the 1st grade materials when we were done with K (we're still using the 1st grade things while he's in 2nd grade and he loves all the Bible and he learns or reviews important phonics when we come to words that are difficult for him).
You may find at some point that homeschooling saves you far more in public school costs than the few hundred you spend on curriculum (my grandson's school supplies were about $100, and he has to purchase clothing according to the school uniform, activity fees, teacher gifts, extra grocery store trips for school lunch contents, and more). At some point you may see room in the budget to add 1st grade, and you can just begin that at any point during the year, or next fall if things are going well with using K.
Here is the author's advice on how to combine K and 1st programs: http://board.mfwbooks.com/viewtopic.php?f=8&t=191
You will notice that you shift the emphasis each year, in order to make it fresh for the younger child who repeats the year when ready to begin his or her own K program.
Here is combining K and pre-K: http://board.mfwbooks.com/viewtopic.php?f=8&t=642
The Kindergarten archives also have several threads about "choosing K vs. 1st" (the archive is listed alphabetically): http://board.mfwbooks.com/viewforum.php?f=1
and the 1st grade archives have threads about "choosing" and "combining": http://board.mfwbooks.com/viewforum.php?f=3
I hope these links help you while the boards are fairly quiet during this holiday season. Bless you as you plan for your precious little ones,