stock Vs stock, the 8p will be a better all round car, no doubt about that, part of that due to its age, which relates to condition/health, and also the technology/evolution of engines.
Remember the S3 has always been a great alrounder, jack of all trades, master of none, no matter what generation you go for.
BUT, as enthusiasts we tend to look at cars differently, i do anyway, for me its all down to the SOUL
of the car, in which case i think the 8l has more, stock or modded.
Its such a difficult debate, and if one had say 15k for example, it would be a hard choice, as an 8l s3 owner, i would find that decision damned hard!!!
You need to answer what you want out of the car in the long run.
Are you the kinda guy that buys a nice car, chooses subtle mods, then sells up every 2 years to keep with something fresh and modern.
Or are you kinda guy who could buy a car, 8l or 8p, and keep it for some years to come, without worrying about loss of value, and to have as a project/love affair.
An 8l will require some chassis modifications and a simple remap at least to keep up with the 8ps power and chassis. But when sorted, the 8l is great.
What would perhaps sway the decision for me, is 'modern' car vs 'old' car. IMO old performance cars, talking pre-2000 design, have such a good feel to them in terms of controls/steering/feedback, that NO modern car can recreate with their electronic power steering, feather light clutches, and lifeless steering feedback.
One tough decision to make, but i think ultimately whichever you go with you will not be disappointed. I will always favour the older 8L i think, im just an old car/classic kinda guy.