I haven't owned either Inspiron of Alienware, preferring ROG myself. And I've only gotten a laptop last year (since I much prefer the capabilities of desktop PCs), so keep that in mind if you read the following.
First off, which Alienware version / which Inspiron 7000 model are you getting? Laptops have different versions for each model.
Alienware 13 has the base model, along with R2 and R3. Alienware 15 has the base model along with M15x, M15x-R2, R2 and R3. Alienware 17 has the base model along with M17x, M17x-R2, M17x-R3, M17x-R4, R2, R3 and R4.
Now, most of the earlier versions have been discontinued, but some stores may still stock them. The latest version of Alienware 13 and 15 is R3. For Alienware 17, it's R4.
As far as Inspiron goes, they have a lot of 15 7000 versions. You have the 7569*, the 7559 (earliest gaming version), the 7566, the 7567, and the 7577 (latest version).
*This is the only 15 7000 model that isn't part of the Pandora Gaming Series from Inspiron.
The Inspiron 7000 (7559 model) isn't as good as the Alienware 13, 15, and 17 versions, but if you're planning on getting a previous version of Alienware, then you might want to choose Inspiron instead. However, that also depends on if you're getting a laptop right out of the box, or if you will be choosing your specs.
I'll give you the rundown of the specifications between Alienware 15 and Inspiron 7000 (7559 model), both of them being the cheapest ones of their lines, and with the same screen size.
Wikipedia also has information on Inspiron (https://en.wikipedia.org/wiki/Dell_Inspiron#Inspiron_7000_Series) and Alienware (https://en.wikipedia.org/wiki/Alienware#Laptops).
If you want, there's also PDFs detailing the specifications of certain models.
If you're not sure about which model you want, then you should think about what you need it to do. The Inspiron 15 7000 7559 starts at $799 and the Alienware 15 starts at $1200, and those are at minimal specs. Depending on how outdated the minimal specs are, you're looking at adding about one or two thousand dollars to the bill.
You say you want "faster graphics", but are you willing to sacrifice quality for framerate, or do you want high FPS and superb graphics? The newer Nvidia GeForce GTX 1080Ti graphics card costs $699 alone and the Titan X is $999.
Large memory is also going to cost you more. The minimum RAM that any laptop is equipped with is 8GB. The maximum is usually 32GB.
Not to mention the Hard Drive space. You might think 500GB of space is a lot, but it fills up really fast. I've had my laptop for a year and don't have nearly as much on it as I do on my desktop PC and yet, I've already filed up around 521GB on it. And what about the drive type, do you want a Hard Disk Drive or a Solid State Drive? SSDs are faster and smaller, but more expensive than HDDs. You could get 1TB of HDD for the same price as 256GB of SSD. SSD also uses less power to write data, therefore increasing battery life and creating less heat.
As far as which laptop has a bigger display, you'll find that the model names indicate screen size. Inspiron 15 has a 15 inch screen and Inspiron 17 will have a 17 inch screen, same goes for Alienware 15 and 17.
However, larger screens decrease portability. The heavier the laptop, the harder it will be to carry around. And be prepared to buy a cooling stand for gaming laptops, since they heat up like mad. Definitely burned my lap a couple of times, but they're great in the winter...
Gaming laptops have lower battery life as well. That's expected though.
For desktops, I highly recommend building your own (if you can afford it). It's what I've always done and my computers have lasted over six years each. If you want one out of the box, then I suppose I'm not the right person to go for that, unless you want me to dig around and pull specs for you to compare.
Plus, you'd need to think about how far later you're looking to get a desktop. Chances are, there will be newer versions that might be better than whichever ones are currently released.
Looking at cellphones, I think you're asking for too many features, considering what they are. They're not computers, they're mobile phones, and they're never going to be as fast or as powerful as PCs in comparison.
Personally though, I've never liked iPhones and Samsung has been my phone of choice, mainly because they have more functionality without needing to jailbreak it. If you regularly clear out your cache and download everything onto an SD card, then you shouldn't have issues with slow performance on either phone.
I don't really like tablets, so I can't say anything on that.
I hope some of my insight might help you, though I haven't had any personal experience with the brands you've mentioned.