She's right, you do get 2 more inches with the Regent. Recaro Comos and Signos reportedly have 19 inch top slots, Regents have 21 inch top slots. Britax gives you much more realistic estimates of when your child will outgrow the seat, but it is just an estimate. Some kids will hit the top slot at a shorter height than other kids, it just depends how your kid is built and whether they're all legs or all torso. Take the Safeguard, for example. They say it will harness a kid up to 59 inches. Those are 19 inch top slots, people! That would have to be one strangely built child. Possible, maybe, but very, very unlikely. DD has much more growing room in her Regent than in her Safeguard.
As far as Regent vs. Recaro, it's a tradeoff. You can get the one that will last the longest, or you can get the one that is the easiest to maneuver. Only YOU can decide which you want! Personally, I don't want the mini lazyboy taking up all the space in my car just yet. Yes, someday, it will be necessary, but not now! But that's just me!