Newer is always better, right? A caveated answer is probably required. In the T490 / T480 context, both machines have their own strength. The T series is Lenovo's workhorse range, it goes through refinement on a regular basis - though it helps to select for what you need.
Advantage of the Lenovo ThinkPad T490:
Marginally faster internals (Core i7 with a marginally faster TurboBoost)
Rapid Charge Support: up to 80% charge in 1 hour & (improving external USB-C battery prices)
Upwards firing speakers for better audio
Softer palmrest feel, easier to open the lid
Thinner & more compact chassis & narrower bezel
More display options (DolbyVision 2k HDR / FHD Lower Power panels)
The lack of a swappable secondary battery will have a workaround (which might suit some), in the form of USB-C external batteries: 3rd party 45W / 65W option start from just over £50. The Lenovo option will be more costly, though definitely works (Lenovo 14000mAh Portable Power Bank Charger; MPN: G0A3140CWW, online from around £130). As a bonus: T490 will support "Rapid Charge", so perhaps you can carry a few extra USB-C batteries, instead of ThinkPad swappable batteries. Depending on your region, the USB-C option could sometimes work out cheaper. Still, the T490 feels as if it was intended to replace the T480s (which didn't have the dual battery system).
Advantage of the ThinkPad T480:
2x RAM slots for up to 64GB RAM (instead of up to 48GB on the T490)
Dual Battery system: 1x battery could be hot-swapped
Retains the full SD Card slot (T490 has microSD)
Been tried and tested for longer
Slightly easier to use privacy shutter
Some people might have modified their T480 with the HDR display. Though this is not a recommendable option from our perspective & will impact the warranty. It's partly because the T480 has a fragile double-adhesive bezel which makes the screen replacement challenging to perform (without damages) & it is sometimes tricky even for the technicians with the correct replacement bezel/plastic parts.
Sum-up:
The T480 might be seen as a "traditional" Lenovo ThinkPad, with visibly more upgradability than its successor (e.g. aftermarket option for 64GB RAM outside of a "P" series; dual battery system, etc). The T490 will be the modern refresh, with more focus on the here and now (DolbyVision Screen option, upwards firing Speakers).
Both systems are quite robust & well made. Select for the use case &the budget would be a sensible suggestion. Some might rate the newer T490 more; whereas others might prefer the T480 (until the next-gen Lenovo ThinkPad T14 arrives).
Just as a context: another "traditional" T series model in the recent time, might be the ThinkPad T440p (swappable CPU/RAM/HDD; easy IPS upgrade option, etc). We definitely appear to be moving to the direction of lighter laptops, with fewer user upgrade optionality; though some might note, that if the other aspect of the design/function/performance is decent, then that's good enough.
