I don't have any recent experience using the latest Inspiron laptops but I have owned 3 Inspiron laptops in the past, bought 2 more for close relatives, and just bought the new Inspiron 14 2-in-1 (5410) for my wife yesterday. Based on her requirements, I chose the Inspiron 14 for its size, light weight, and selected the i7 CPU to get Thunderbolt support so that the laptop can be connected to an eGPU (Coolermaster EG200, GPU tbc based on budget) box for some gaming later on. The one I bought will be pre-installed with 16GB RAM (the only option available based on the CPU I selected), and 512GB SSD, which I will be upgrading (DIY) to a 2TB WD Blue SN550 SSD.

In general, I have had good experiences with the Inspiron series. I would say it's pretty good, but not great, though that is subjective as it really depends on what your requirements and preferences are. The light weight was a major requirement from my wife, and the Inspiron 14 is around 1.3-1.4kg vs my Alienware's 2.6kg. She took one look at my Alienware and said, too heavy!

My favourite Dell laptops have been an XPS 13 (old one, long ago), and my current Alienware 13 R3. I have also owned an Alienware M11x R2 (too small for my liking), and an Alienware 15 (amazing 4K screen). I am about to get the new Alienware X15, so my preferences are a bit more towards the gaming side.
I think it does, when I was doing my research for my wife's Inspiron, I was searching online for Inspiron 14/15 RAM upgrade videos, and found a few that showed how to upgrade the RAM. I would recommend finding out the specific model number of the Inspiron 15 (If you spec the laptop on Dell website and go to checkout, but before payment, it will show the exact model number, which should be a 4-digit number like 5410 for the Inspiron 14, or something like that). Then search for Inspiron 15 RAM upgrade videos and usually they have the exact model number in the  video name. Then you can see the inside of the laptop to see if there is additional slot or not.