On-line UPS's have the benefit of removing transfer delays, but that's generally not that important in general server environments (more for medical, or cases where your load doesn't have enough capacitance to handle a couple missed cycles, which most server PSU's have more than enough to handle that).
On the down-side is that you go through batteries faster due to their constant use, normal UPS's you want to replace them 3-5years with on-line if going direct to battery (opposed to super capacitors & batteries) probably 1-3 years depending on use.
Yes, XJ-SA24-448R-B's and similar are a bad design. STK was using that design in their SANs back about 5-8 years ago and was no end to problems with them (mainly drives falling offline, mating problems et al). To get around the issue of pulling multiple drives, STK had a dual-head design so the front drive was controlled by one raid system and the back drive by another storage processor. You would have to offline both A/B drives and wait until it was synced to a hot spare before you pulled the blade. Slow and yes if you had another problem at the same time you're kind of screwed (they were only doing RAID3/5/[1+0] back then).
The later ones you picked 48 drives vertically mounted in a 4U is a copy of the SUN Thumper systems, which are good for high density but air-flow is an issue due to the amount of restriction in the cases, plus to replace a dead drive you have to shut down/pull the entire unit out of the rack to get to them. To mitigate that you need a decent number of hot spares in there so you can do replacements in standard outage or downtime windows. I normally do a 1:10 - 1:15 ratio depending on quality of drives (consumer or enterprise). Another item is how those chassis are wired, the ones you linked do not show internal wiring schematics at all. You want to avoid situations where, like I mentioned before, a bad drive could take down an expander and all other drives attached to it. This is why I opted toward multiple chassis et al. It comes down to your HA requirements and what you can live with in cost.
Just FYI, SM is supposedly working on the issue I had with the SC837E* chassis and the LSI HBA's but no solution yet. I've since went with plan B here and just purchased some more of the AIC chassis. Not as good as a density solution but couldn't wait for SM (considering that they use the LSI2008 chip on their motherboards this implies that they didn't do much QA testing).