The Evenflo Horizon is discontinued.... I think it's important to also mention here that most seats expire within 6 years from Date of Manufacture & OverHead Shield styles are not as safe as regular 5-point harness straps >
http://cpsafety.com/articles/fivepointshield.aspx
Usually, the most ideal features to consider are seats with low bottom harness slots + high top harness slots (to keep kids harnessed longer) but a small vehicle may require a smaller
convertible carseat -- this means I would plan on saving towards high weight harness options to purchase after your kids outgrow the
convertibles (too many kids are just not yet ready to use a
booster by then). My website explains why & provides links with such options to consider for future use, although hopefully by then there will be more options, too!
The Evenflo Triumph, Safety 1st Uptown &
Britax Roundabout or maybe even the
Graco Comfortsport come to mind as comfortable, easy to use, short
convertibles. See if you can test them out at a Babies-R-Us, etc. or make sure that the online retailer has a good return/exchange policy. The most important thing is that the seat is used correctly: installation + buckling. Your local CPST can help you achieve correct installation (ask to see their current SafeKids certification card).