When I got mine about two years ago, I was shocked at how hard it was to find a LATCH booster seat. Recent research shows that a rigid LATCH system is more important than a high-back, even. I also needed a narrow base to fi tin my smallish car. After searching on line and in person almost everywhere, I found only one. I got the Clek Ollie (ordered them online). It fits in my compact sedan and in DH's SUV latch systems perfectly, and has an easy release pull tab on the front. I love its size and simplicity. There may be more LATCH boosters out there now (I hope there are anyway) but I am very happy with the Clek Ollie.
http://clekboosters.com/p-137905-clek-2011-olli-booster-seat.aspx#!vID152315
ETA: just wanted to add that the rigid latch is so much simpler than the belt tethers on car seats. It literally clicks in and out in seconds.
ETA: Okay, little miss knowitall, here you go...
It goes without saying that if the seat is covering where the belt goes it's not safe. I'm assuming Sugar and Spice is relatively intelligent to figure that out. the BENEFIT of the Ollie is that it is narrow and does NOT cover where the belt goes. Also, the BENEFIT of a rigid latch is that it cannot be installed incorrectly, or too loosely as is the problem with many "soft" belt latches in car seats.
The Clek Oobr offers a high back and a rigid latch but is out of her price range, which is why I did not rec it. But if you have to choose one over the other, the LATCH is your best bet. The lathc-less ones have too much side-to-side "give' in the event of an accident, causing improper positioning of the best as the seat moves, even a small amount. And the belt position plastic clip is completely useless in the event of a high-impact crash (there is research on that as well).
Also, the Clek is specified for 40 inches and 40 lbs. No child under that height and weight should be in a booster.
ETA:"The results of this study, though still preliminary, suggest that protection, specifically of the chest, may be enhanced if the booster seat is anchored to the vehicle seat, as one would attach a CRS. Use of the LATCH and tether produce a more effective coupling than typically produced by the vehicle’s lap/shoulder belt alone or in conjunction with a high-back booster without LATCH."
http://www-nrd.nhtsa.dot.gov/pdf/esv/esv19/05-0258-O.pdf
"The measurement of how far forward the child's head moves in a crash is called "Head Excursion."
Since Sept. 1999, all forward-facing child safety seats must pass a stricter head excursion limit of 28 inches. Most forward-facing child safety seats cannot pass this 28 inch limit without using a tether to hold the top of the car seat back. None of the high-back boosters (that weren't part of a convertible car seat) currently come with a tether strap.
(basically that means that the high-backs without a tether (which none of them have, even the latch ones) it is no better than a backless.
http://www.thecarseatlady.com/index2.php?option=com_content&do_pdf
There are more, but I don't have time right now to link them all for you. Look for research from 2012. That is going to give you the most up to date info. Also, look for actual studies, or peer reviewed scientific articles, not blog posts or discussion boards.