Moving from a 630, what justifies the price tag of the 955 ?

0 difference, really?

You mean apart from integrated maps...

Apart from touchscreen

Apart from improved gps sensor (which now has multiband and is better overall)

Apart from better soc/batterylife

Apart from more metrics/features

If it's  worth the price is another question, for me as a runner it's now good enough... But I paid full price and still feel I overpaid. Reason being software support is very disappointing (read around to get an impression of issues and bugs).

But if touch and maps are not important for you I'd reconsider buying a 955, those are THE selling points.

As a data nerd, tons of data...

Maps are not important to me, but I really wanted the training readiness metric that is not present on the 255. There are many other features, like touch, altimeter/barometer/compass, that are not present on the 255 Music, the $100 extra was worth it to me. If none of the extra features are important you can save and get the 255 or 255 music, they seem to be great for what they are.

don't too expensive for a watch full on bugs unstable software with freeze ups and eventually even death, ull end up with waiting for a replacement from the service dep, screw up ur training plan and wonder why TF they launch the flagship running watch without enough testing, (ofc cause its cheaper for u to do so.) either stay away from garmin altogether or stick to a cheaper model. i loved the features and when the watch had a good mood to work, but in the end you will have more reliability on a 9.99 watch from aliexpress. i mean seriously guys (garmin) step up

You can find a second hand 945. That will be a great update for peanuts. 945 has maps, music, garmin pay. 955 is a minor upgrade of the 945. Better CPU, better graphics, better battery use, faster wifi, touch screen. Is it worth it ? Difficult to say. With Garmin if you get stuck with a model for several years the price of upgrading is really steep. And most of the features you won't even value/notice because you didn't saw it improving along the models. 

Bottom line, if you don't want the latest and greatest it's impossible to justify the price. 945 for me was one of the best Garmin watches. I'm happy with my 955 but because I had a 945 and can see the improvements they made. Not disruptive, but even so many areas have improved. But most of the stuff is "nice to have" not "must have", over a 945 of course

You are comparing 2 watches of 7 years gap.

There are bound to be SIGNIFICANT differences.

For budget, just consider the 255.

Still, there are very few differences between the two (630 vs. 955).

If you only consider running metrics, I'd say the 955/255 are even a step backwards, you lose the running dynamic metrics (and the touchscreen for the 255).

The only new features are mostly software driven and will only work, for the most part, if you wear the watch on a 27/7 basis (which I don't).

So those 955/255 are definitely newer models, but are very modest upgrades for anyone with a high end watch purchased over the passed couple of years.

Didn't mention one thing : I'm considering un upgrade from my 630 because the battery is getting weak. Sent an inquiry Garmin's Support asking them how much changing the battery would cost and I just received an answer. They won't replace the battery. That's an unsustainable upsell

955 and 255 use a new GPS chipset. And multiband in order to vastly improve GPS precision.

It's a HUGE hardware change and improvement.

As I recall you require a HRM for Running Dynamics on the 630? I don't notice any "step back" from the 630 to the 955 , quite the opposite. The 955 is much more reasonable priced than the 945 / 945LTE were originally , I was in fact quite surprised at the price point because of that. The 230/235/245 line were always below the 6 series so the current 255 is actually much "higher" up the line than those were as it easily replaces the 630 (apart from touch which many dont use anyway and the 5 button style makes the lack of touch less of an issue really.). I a had the 630 from release until late 2021 , moved to a 745 and in 2022 sold that for a 955.  Very happy with my 955. Yes a few bugs but I think all the watches seem to be buggy at the moment...

I do understand this, but it's just a normal spec bump assuming those 955/255 are 7 years newer.

Still, do I need it ? There's nothing wrong with the GPS precision of my 630. I've been running with a Garmin watch since the 205 and GPS precision has never been an issue.