Looking at either a 2014 Specialized Stumpjumper FSR evo (the alloy one) or a 2015 Whyte 909.
For mainly trail riding and occasional visits to trail centres.
Your thoughts/experiences please.

That’s two very different bikes, shirely ‘you’ need to decide if you want FS or HT. Both are very good, lot of owners love both of those bikes and either will fulfil your trail riding/trail centre requirements.. what do you want before the thread becomes an owners shoot out!

If I knew what I wanted I wouldn’t ask.

I’ve only ridden hardtails/rigids in steel and 26″ wheels.

I’m a short ar5e and not sure about 29″ wheels but I’m not getting any younger (approaching 50) and perhaps looking for a less jarring ride. But FS is a lot further from my comfort zone than a hardtail on 27.5″ wheels.
And the hardtail is lighter and has nicer kit.

Then go get some test rides, FS’s aren’t for everyone, and not all FS are the same. I personally dislike Horst link suspension bikes, but wouldn’t be as conceited enough to say that Spesh bikes are rubbish, they aren’t. I rode a HT for the previous two years, and couldn’t wait to buy a FS’er this year.. going back to the HT for winter, I still know I prefer the FS, but that’s me..
Why rely on strangers to make a decision for you, go bite the bullet and buy one, and if you hate it, sell it buy another! I’m not saying forums are pointless, but let them guide you, not make your decisions for you.. 😉

MTBing is about make decision with your heart, not your head, value shouldn’t be totally ignored but it shouldn’t be the only reason you buy a bike.

Sounds like a short travel 29er would be ideal for what you’re after. Something like the Whyte t129 would be great.

Anything over 120mm travel is going to be pointless for the riding you’re doing, and if you’re after something a bit more comfortable, then a hardtail isnt a great choice. Don’t get too hung up on weight, as you’re the heaviest part of the bike, a couple of lbs doesn’t make that much of a difference at all.