I think the point being made was that having a 98% or 91% mon is, in probably 999 of 1000 or more scenarios identical and that people put too much stock in having a hundo. I’d love a 14/14/14 shiny, it’s going to do the same job as a hundo in all but the most extreme of circumstances. IV’s roughly, make less than 10% of the difference between 0-45 and so, having one that’s 42/45 is such a fine margin below perfect, it really is imperceptibly different. Especially so in these days of remote raiding where it’s far less likely that you’ll be short manning a raid - unless that’s your thing and so DPS is even less important to most people than it was, say, 6 months ago.
Whilst, in a world of infinite time and energy we could and probably would all hunt down 6 or even 12 hundos of each type, this is a game of time and resource management. I was lucky to have 4 lucky rhyhorn to evolve on CD so my first 5 in my rock team are lucky rampardos, 4 lucky rock wrecker rhyperior followed by a 1/5/7 rock wrecker rhyperior. It performs, ever so slightly, worse than the 4 before it but when it was in my Moltres team, that Moltres didn’t enjoy the rocks from the final rhyperior any more than the previous 4. Until December rerun day, there’s nothing else going that will perform better in that 6th slot than that rhyperior and so, it stays there.
Size gets you a couple of badges for ratata and karp but apart from that, whether size was intended to matter at some point and it just never happened or if it’s just a point of interest to bulk out the appraisal dialogue bubbles, unless you want to make a team of “all XL” giratina, it’s purely aesthetic.
Personally, I’d choose the shiny, because it looks cooler. It will perform just as well as your non-shiny.
That said, I love Tina-O so I’m trying to build a team of 6 shinies, regardless of IV - some people would say that spending well over a million dust (assuming I can’t lucky trade a few of them) and 1000+ rare candy on one type is nuts, but I don’t care, it’s what I want to do. Play the way that makes you enjoy it most. If you enjoy having the best IV, go with the non-shiny 98%, if you enjoy shinies, go with the shiny.
Ultimately, Tina-O isn’t a necessary counter to itself and it’s here for another week. If you’re not desperate to max one now, you could land an XL shundo between now and next Friday night that solves your entire dilemma.