I'm sorry if my response sounded a bit snarky, I really wasn't trying to be, I was just pointing out that 64GB doesn't go very far. As for what universe I'm in, I'm in this universe, where the average iTunes library size has been reported as over 7,000 songs. Granted that stats probably skewed by people with very large library's, but I would still suggest that your 150 song library is much smaller than the average. If you can get by with an 8GB that's fine, but I'm just trying to point out to the OP that if they go for a smaller capacity they will probably regret it in a year. I know several people that have said like you when buying an iOS device that they would just be paying for storage they won't use, and then they end up having to buy another device a year and a half later because the last one is full.
Also, you might want to re-read the OP's initial comment where they stated
"Thinking largest size is way to go to future proof Ipod Touch with new apps coming out each day"
Lastly, the OP never suggested they wouldn't fill it up. They asked if the size would slow it down or make things hard to find [which it doesn't in my experience], so suggesting that they would just be wasting money on capacity doesn't really answer what he was asking.