Fenix 5S as a runner's watch?

For strictly running, I don't think you're going to gain anything by upgrading to a 5s to be honest. You're getting the optical HRM with the 5s, so you technically won't have to wear your chest strap anymore for heart rate. You'll still have to wear the chest strap for the advanced running metrics, however.

Here's a side by side comparison, courtesy of DCR: https://www.dcrainmaker.com/product-comparison-calculator?type=watch&ids=69229%2C51129#results

Edit: DCR recently did a review of the Apple Watch 2. As a running watch, it's pretty lacking. I think you'd be very unhappy going from a 630 to an Apple Watch 2 for running purposes.

There are some potentially interesting advances in the F5 (and normally coming to the F3) for running, namely Training Load, Aerobic/Anaerobic training and TE v2. We'll have to see how useful they are though...It doesn't seem anyone has seen much value in the Lactate Threshold feature for instance.

If the oHR is as poor as it's become on the FR235 don't count on doing without it for reliable HR data/analysis.

There are some potentially interesting advances in the F5 (and normally coming to the F3) for running, namely Training Load, Aerobic/Anaerobic training and TE v2. We'll have to see how useful they are though...It doesn't seem anyone has seen much value in the Lactate Threshold feature for instance.

If the oHR is as poor as it's become on the FR235 don't count on doing without it for reliable HR data/analysis.

Is the oHR the same as on the FR735? I heard that is quite good.

From what I've read, the oHRM is a slightly newer version to enable them to do almost constant resting heart rate polling (every 1-2 seconds). They claim it will be "as good, or better" than the 735 for heart rate metrics.

To be honest, I would hold out until it's released and been tested to see how it performs.

My FR235 is quite good for me personally, but I know it's not great for some folks.

The OHRM is a newer generation than the 235 (and I believe the 735), and it's also much flatter (no bump) so should be more comfortable.

And if you do use an external strap, there's also the support for BT HRMs and not just ANT+.

For strictly running, I don't think you're going to gain anything by upgrading to a 5s to be honest. You're getting the optical HRM with the 5s, so you technically won't have to wear your chest strap anymore for heart rate. You'll still have to wear the chest strap for the advanced running metrics, however.

Here's a side by side comparison, courtesy of DCR: https://www.dcrainmaker.com/product-comparison-calculator?type=watch&ids=69229%2C51129#results

Edit: DCR recently did a review of the Apple Watch 2. As a running watch, it's pretty lacking. I think you'd be very unhappy going from a 630 to an Apple Watch 2 for running purposes.

It would be nice to always have optical HR as I won't always wear the chest strap, and never during a race.

I assume the barometric altimeter in the Fenix 5 will give more accurate elevation data for a run than the GPS-only 630? I note it's made from stainless steel - will the extra weight be noticeable? Which of the optional straps is better for runners?

I've never had a problem with the "bump" of the oHR on my FR235, especially since it should help create a "seal" for improved readings. Can't see it's removal helping to improve oHR reliability on the F5 but we'll have to wait for first real life reports.

There might be more than hardware issues on the FR235 as it suddenly became very unreliable with the v6.10 update in the fall. Having said that the VAHR I borrowed recently was pretty poor as well for HR...

It would be nice to always have optical HR as I won't always wear the chest strap, and never during a race.

I assume the barometric altimeter in the Fenix 5 will give more accurate elevation data for a run than the GPS-only 630? I note it's made from stainless steel - will the extra weight be noticeable? Which of the optional straps is better for runners?

The altimeter "should" give you more accurate data, but I know there have been some major issues with the altimeter in the Fenix 3/3HR.

The weight will be a little noticeable, but nothing extreme:
FR630 with silicon strap - 1.6 oz (44 g)
Fenix 5s with silicon strap - 2.4 oz (67.0 g)

Straps come down to personal preference. I've always just used the included silicon strap. The optional straps are either just extra colors in the silicon variety, or the leather and stainless straps, which are more for casual wear.

A lot of good responses already but I'll add my own 2 cents.

Weight: You'll notice it at first but once you get used to it, it won't be a problem. I went from FR235 to F3 without issue.

oHRM: That they've gone to more frequent polling with the 24/7 mode says more about getting battery life out of it than accuracy improvements but one would hope that they've learned a lot over the last year and a half that they've been making their own sensor.

Elevation: Having barometer based elevation is better than map based but even with a properly functioning unit you can still get some wonky results. Since the elevation reading is tied to the temp sensor as well as the barometer a shift in temperature over the course of a run can make things look off. I run in the evenings so this time of year there can be a big temperature change between when I start and end my runs which can lead to it looking like a net elevation loss even though I started and ended in the same spot. The opposite if the temp goes up while you're running.

I'll probably go with the 5S myself because I have skinny monkey arms too, but I'll be waiting for a while to see how well it works out of the gate and let the growing pains (if any) get worked out.

A lot of good responses already but I'll add my own 2 cents.

Weight: You'll notice it at first but once you get used to it, it won't be a problem. I went from FR235 to F3 without issue.

According to the Garmin site, it's 67g for the F5S vs 44g for the FR630, but the HRM chest strap alone weighs 60g, so there would be less weight overall.