I have both an 08 LS and 08 RX. Valley is right. The RX is good for transporting, but the ride is night and day. Once you drive an LS, it's hard to feel good driving much else. Not that the RX is a bad vehicle, but the rougher ride becomes a lot more noticeable. I suppose if you're going to use the SC instead of the LS, it's not a bad trade.
I have 08 LS460 and my wife has 10 RX350. Go for Rx350 if you need more space. You can fold rear seats down and put lotta stuff. Rx is rougher compared to LS. It's not smooth as LS. I love both. I love Rx for the space and trunk space and it sits higher than other cars. I love LS for nice smooth comfortable ride. (LS engine sounds lot more quieter than RX but I can't say that no more because I have exhaust on my LS ) On freeway, both cars are smooth and quiet. RX just makes more noise when you step on it compare to LS. I like the mouse scrolling type GPS on RX and double pane windows for extra quietness. Watch out for double pane windows because sometimes inside rubber folds inside and makes creaks every time you roll down the windows. Dealer took care of it but check for creaks when they have double pane windows. I took both LS and RX for long trip (+1000 miles) and both of them are really comfortable. So choice is up to you. If you want sedan, more luxury, smoother ride then keep LS and if you want SUV for more space, go for RX. So far no major issue on both. Only replaced control arms on LS. Other than that, just good ol regular maintenance~