Does anyone have any suggestions for a good portable charger for Dell XPS 9370 and 9300 laptops??

@julap7  XPS 13 systems are designed for a 45W power source.  They might work with lower wattage sources, but you may see slower battery charging and/or reduced performance.  I personally just bought this charger, which you can find on Amazon at least in the US (not sure where you're located).  Thanks to its use of GaN technology, it's a lot smaller than other similar wattage chargers, and the fact that it supports 90W output means it will work with a wide range of other devices, which might end up being handy given that USB-C is rapidly becoming the predominant device charging standard.  Even better, it can split that 90W across its two ports in various ways: 90/0, 60/30, and 45/45.  So actually in your case, you'd be able to run/charge BOTH of your XPS 13 systems simultaneously at max performance from this single charger.  I don't have a 9370, but I do have an XPS 13 9350, a Latitude 7480, and a few other Dell laptops, not to mention other USB-C devices like an iPhone Pro and an Oculus Quest.  This charger works flawlessly with all of them.

@julap7  I forgot to address your question about whether USB-C cables matter, and I figured I'd give you a bit more guidance about chargers.

First, the answer for cables is they can make a difference, but wouldn't for just the XPS 13 systems.  Most USB-C cables are rated to carry up to 60W of power (3 amps at the 20V max of the current USB PD spec).  Some USB-C cables are rated to carry up to 100W (5 amps at 20V) because they have thicker wiring to carry that extra current safely.  So if you want to use that 90W charger to provide 90W, you'll need a 100W cable.  But if you're only dealing with XPS 13s that will only draw 45W max, then you don't have to worry about this.  For what it's worth, for cables up to 6 feet, if you only buy cables that are designed for USB 2.0 data and power (as opposed to USB 3.x and video), then 100W cables don't cost much more and aren't much thicker than 60W cables, so I tend to buy 100W cables as standard up to those lengths, unless I know I'll only be using it for lower power scenarios, like wireless charging pads or something.  But if you get 10-foot cables, 100W cables are (at least currently) quite noticeably thicker than 60W cables, so I wouldn't get those unless you need more than 60W support and/or you'll be "parking" that cable somewhere rather than carrying it around, in which case the added thickness won't matter much.  And returning to a point I made earlier, getting cables that only support USB 2.0 and power will save you money and thickness compared to getting cables that support USB 3.1 Gen 1 or 2 and video output.  I basically buy USB-C "charging cables" for scenarios where I know I'll only use them for charging, and more expensive USB-C "data/video cables" only for scenarios when I know I might use them for that purpose.  (The data/video cables can of course be used for power as well.  And some of those support up to 60W, while others support up to 100W.)  If you really want more gory details, I wrote an explainer post about the various differentiators of USB-C cables here.

And then in terms of the charger specs to look for, if you don't go for the one I linked but want to make sure you get the right product, I'd suggest getting at least a 60W charger because they're not much larger or more expensive than lower wattage chargers, and 60W will cover many electronics you might want to use.  But 90W is better, especially if you look at multi-port chargers.  On that front, be aware that with multi-port chargers, you have to look at the maximum output available from a single port and the maximum total simultaneous output across all ports to get a sense of how you'd be able to use it.  And some multi-port chargers will only split power in certain ways.  For example, I have a 60W dual port charger that can provide 60W out of either port when used solo, but when both ports are active, it can only do a 30/30 split.  It couldn't do a 45/15 split, for example, which limits some use cases.  Again, that's why I like the 90W dual port charger I linked, since it can split power output multiple ways.  The other thing to look for is the output voltage levels it supports.  5V is mandatory, but you should also make sure it supports 9V (used for fast charging some smartphones), 15V (used for fast charging some tablets, other high power portable devices, and some lower power laptops), and 20V (used for most laptops).  If you can find a charger that supports 12V as well, so much the better, although that isn't as common.  But for example if you get a charger that only supports 5V, 9V, and 20V, you might find that you can't fast charge certain devices that will only fast charge at 15V.  Those devices would likely still charge at a lower voltage, but would do so more slowly.  Or if for example you got a 45W charger that only supported 5V, 9V, and 15V -- but not 20V -- then you might not be able to use it at all to charge most laptops.  Many laptops will ONLY use 20V, so they wouldn't use that 45W charger even though they might accept that same 45W from a charger that could provide it at 20V.  Lastly, while many electronics will accept lower wattage power sources than they're designed for, with the tradeoff being slower charging and/or reduced performance, there's usually a limit to how far below their "target" wattage they'll go.  For example, I have a laptop designed for 135W.  It has a proprietary charger connector since that's more than can be provided over USB-C, but it does support USB-C charging as well.  However, it will only work with USB-C chargers down to 60W.  It you connect anything lower than that, it won't even bother trying to use it; it will just ignore it, not even acknowledge that a power source is connected, and keep running on battery power.  (And even 60W results in practically no charging if you're actively using the system, since that's less than half what it's designed for.  But 60W works fine for overnight charging.)

Hopefully this helps!