DH and I are on the market for a new carseat (or two...)

DS is nearly 7 months, 18 lbs and 27 inches. Our infant carseat goes up to 35 lbs and 32 inches. Based on his height, it appears that DS will probably surpass the height before the weight maximum. At this point, I do all daycare drop offs but, if something comes up and DH needs to pick him up, we are SOL. Our daycare also can't store his infant seat there due to California daycare laws (at least, that's what she said.)
So, we were thinking about getting a convertible carseat, especially since Target has some on sale for 30% off.
I would like to get something that will work well in the future and (possibly!) for another LO in a couple of years.
Anyway, do we get two? One nice one and one okay one for the car we use less? Get one and move it back and forth?
Any recommendations for a nice, but economical convertible carseat? I have no idea where to begin...

Also, sorry for all the unnecessary info, I just figured I would "throw it all out there." Thanks in advance!