My wrist is on the small side and I've always gone for the larger 44mm.
The size difference isn't so immediately apparent when on your wrist. And that little extra real estate is really helpful in navigating that screen real estate, especially if you get fat finger symdrome.

I've never gone with a case as I think it takes away from clean facade but that's more personal preference so I can't comment on the impact a case would have on the profile apart from changing it to a mini G-shock. Been wearing the watch sans case for full on sports like futsal and all for 3 years now, touch wood still going.

I use the regular Sports Band for sports. Easy to wash off the sweat and dry after.
For normal use, I'm using the Sport Loop.

For what it's worth, I have no reason to think Apple watch straps are not safe.
Among me and my friends, I have never come across an instance of strap failure. Swimming included.

I have 2 Sport Loops for casual wear and 1 Sport Band that I use for sports. Same experience across.
It's all originals, no experience with third party straps.

I prefer the loop for daily use due to it being fabric, so there's comfort and some airiness.
In terms of reliability, it would say it has been every bit as reliable as the band. So yes, very reliable.

One of the loops I have is 3 years old now. The other is about 1 and a half.
Just checked the velcro on both for comparison, the older one adheres just as well. I don't think it has become weak at all.

The newer loop I have is a Nike sport loop, in black with reflectors. Classy and it's my daily choice.