I have the T420 and the T420s. You already know the differences between the two laptops technically speaking (slim one versus fat one, different battery system, upgradable CPU in one, not in the other, USB 3.0 on s model, USB 2.0 only on T420, that are the mainly differences between the two machines).
From the user point of view (and it's naturally a totally subjective point of view) : I adore the T420. I don't love the T420s (I can't say I hate it, if so, I wouldn't have bought it). I have a T400s and a T410s, they are not perfect, but it's nice to have thin machines with the 16:10 ratio on the display. Their big default : 1.8" microSATA drive. They are expensive and hard to find. It's better to use an 1.8" microSATA adapter to mSATA nowadays. The T420s corrects this, but it's also a 16:9 machine and I have always thought it was too large compared to its thinness. You are supposed to have a very portable machine, and a one you can use hours and hours on battery. Yes, it's thin, but the autonomy is not great (even with the second battery – which deteriorates quickly) and it's not as portable as the T4*0s. I can put the T4*0s in my bag, I can't put the T420s. I have a bag for the T420 and many other bags for other machines. But I don't want to put the T420s in the T420 bag because it floats (« Everything floats down here !
»… Dang ! Wrong topic !
). And I don't want to buy another bag again !
With better batteries and 16:10 ratio, it could have been a very nice machine. For me, here are the real problems of the T420s.
The screen isn't fabulous neither. But it's common with Lenovo. Bad resolutions, bad TN panels, I have learned to live with it. But again, on a premium and portable machine, the display is important. You're outdoor and you see nothing on your screen, what can you do with your PC ? Not a lot, I fear.
I think Lenovo should have done a PC with less compromises. It would have been more expensive, but when you have 2K $ for a laptop, you can pay 3K $ !
Well, not necessarily but I think that if you want to buy such a machine, you don't want a laptop which isn't perfect on every aspect. You're willing to pay a high price for a perfect machine. Here, you still pay a lot, but you don't have all you could expect from this type of machine.
The philosophy of the T420 is different. Less half choices. You can have a lot of power, the laptop is heavier, but if you need power, the weight isn't always your primary concern. The machine is more upgradable. You can go quad-core (but I wouldn't recommend it, as the laptop will be very hot and is likely to throttle). The i7-2640M I have in mine is sufficient for a lot of tasks (multimedia, surfing, a little gaming). And, well, it comes with 16 GB of RAM too, it helps ! You can only have one battery, but one hell of a battery ! 9-cell with which you can have 13 hours without an electric wall socket close to you. For a 2012 machine, it's veeeery nice ! And battery-wise, you could go for a specific T420 battery, but they tend to get old. I chose to buy a T430 one. The reference isn't the same exactly and they are, of course, more recent. The batteries can work on T430, T420 and T410. You could use T410 and T420 batteries on the T430, but you couldn't charge them. So, go for the T430 batteries, and you'll have an efficient solution for various laptops. I bought my 9-cell on january 2019, a Lenovo original one, I paid it 90 €. It was from 2015 and perfectly new. As of today, it show 10 % of wear and the autonomy is still great (between 7 and 11 hours depending on what I do).
As I'm an hotliner, I love to get dirty with my laptops. I loved the fact that I could build my T420 virtually from scratch. I upgraded everything (screen, RAM, CPU, keyboard, disks, battery, touchpad sticker) for next to nothing (the laptop, supposedly dead but finally not) cost me 50 €. I already had the battery and the RAM, but the other components cost me 75 €. A very nice bargain as I have a fully loaded laptop – except the CPU, I didn't want to go quad core on this model because of the heat and the throttling as i said earlier. Quad Core will be for the W520 I received last week, the 2860QM is en route and should be mine between the 20ᵗʰ of august and the 10ᵗʰ of september !). My precious !!!
It's nice to have the T420s in my collection, because here in France, it's not a so common laptop (and I don't talk about the W520, it's even more difficult to find !). And it's still rather expensive. But that's all. Just a laptop in my collection. I use it from time to time, but not so often. The T420, on the contrary, is my daily, I use it in the house, but I can use it also outdoors if I know I need and power and battery life (I like to go out with the T440 also, because of its thinness, its nice screen – I put an IPS Full HD screen – but I don't like its keyboard so much so it's a second choice). And I plan to use the W520 as a desktop replacement (I plan to build a big new wood desk with plenty of room on it – 185 × 80 cm, I will put only what's necessary on it and the W520 will be absolutely outstanding on a nice big minimalistic desk !).