Well, maybe upgrade wasn't the best word to use, kind of is, kind of isn't. The machines are quite obviously based on the same platform, and they share quite a few parts as a result. But there are some notable differences, though these may not be relevant to your specific printing needs/desires.
I've had the P1S for 14 months, I've printed more items than I can remember on it and it's performed pretty much flawlessly for me. Minor issues with some ASA filament, but I'm not convinced it was entirely the printer's fault. Not all filaments are equal and I think the brand I chose might have had something to do with it, though I was able to get it to work by tweaking the machine settings in the slicer a bit. Anyway, in contrast, I've only had the X1C for 1.5 months and only had the time to print maybe half a dozen items total, with PLA, ABS and PETG-HF. I'm assholes and elbows with work right now and haven't had much time to dig into it.
I'll start by saying the P1S is a
very capable printer and can be tweaked to print with most of the same materials as the X1C. Bambu Labs parts are pretty reasonably priced, so upgrading the hot end and extruder gears to hardened steel won't break the bank. I had originally planned to simply add an AMS to my P1S, but the more I read about the X1C, the more I was interested in checking it out. In the end I went for the X1C/AMS combo.
- The displays and the enclosures are probably the first differences you'll notice.
- The P1S display is perfectly serviceable, though the larger touchscreen on the X1C is pretty handy, especially when configuring the AMS directly (via the machine, not the app, etc.).
- The P1S enclosure is plastic whereas the X1C is aluminum, and though I doubt this has any major effect on the printing process, it certainly gives the X1C a slightly higher quality feel. There may be a difference in how the two materials react with internal chamber temps, though I suspect if there is a difference, it's minimal.
- The X1C has an improved camera to monitor prints in real time. Allegedly, the X1C's camera has the ability to detect print failures as well. I don't monitor my prints in real time via the camera on either printer, but the X1C's video is higher quality should I need/want to use it for some other reason.
- The "fancy AI bullshit" (
for the record, I'm not particularly enamored with AI in general). I can't say how much the AI factors or how intelligent it actually is as of yet, but I do think the micro Lidar works well both in bed leveling and first layer checking, a feature not available on the P1S. While not critical to getting good prints, it may well help detect/prevent failures. Fortunately this has not been an issue so far during my limited time with the X1C. She's run like a champ so far.
- Both printers can accept MicroSD cards, though the X1C has an addtional 4GB of internal storage. Probably not a deal breaker or maker for most people but it's a difference and bears mention.
- For me, one of the biggest draws was the difference in max build plate temperature. Running on 110v, there's a 20 degree Celsius difference in max build plate temps (P1S=100, X1C=120). I have found printing with certain filaments, I needed to bump up the plate temps from the default print profiles to solve some print issues regarding adhesion and warpage on the P1S, but was only able to push it up to about 102 (with an inert error message on the display for the print duration). While this did improve the print issues, it didn't resolve them completely. I think the extra temp headroom on the X1C may well prove useful.
I don't think you can go wrong with either machine. That said, if you're only looking to print with filaments like PLA, ASA and ABS, you can produce good prints and save some coin by going with the P1S. It's a damn good printer for the money.
Hopefully the above helps. I don't think I left anything out but there's enough folks here with the X1C, I'm sure they'll fill in the blanks if I did.
Way to go adding a new opportunities for uses of the 3D printer I didn't have in my mind already... 
