Just a note about the waveform updates: it matters a lot when you're trying to find rare, intermittent glitches in a signal, but not a whole lot more. Depending on the implementation, it can mean faster on screen displays or faster update of math functions or whatnot, but that's not really a requirement of the spec.
As for the channel debate, it's really up to your own personal usage scenario. Of course, just having a channel or two is hugely more valuable than having none, and each added channel sort of drops off in usefulness, but I find myself really preferring more in a lot of applications, and a recent repair's probing would have actually been a lot simpler if I had 8 or 10 analog channels, though most of them would only need kHz of bandwidth (and there are data acquisition devices that basically do this). When you're following an analog signal, it's great to have probes on several parts of the signal path, the output, and the power supply rails, so it would be easy for me to find an application for 6+ channels, and more means you will ID correlations faster and have to switch probes less frequently.
Of course, in many of these cases you're dealing with a repetitive signal that you can just look at one part and then another, or it's close enough to DC for a decent multimeter to pick it up, or you can verify that one part of a circuit directly tracks another, so that probing one gets you the info you're after.... but especially if you're designing or troubleshooting analog designs that have several stages or filters, having more channels is definitely nice.
I had a 2 channel Siglent that I liked quite well and who's performance suited my needs... I found a couple uses for more than two channels and found a good deal on a 4 channel Rigol and switched to it. I won't buy another 2 channel scope because of how often I have all four probes doing something, and I'm trying to get other instruments with trend-chart displays (works like roll mode) because of how valuable that real time visible correlation has been. It's not a real requirement for making measurements and debugging, but it's a quality-of-life improvement that I really prefer.