Pastry cream filling - worth getting the expensive heavy cream?

I wouldn't bother with the ultra (expensive) cream for pastry cream. By the time you're finished, the pastry cream is very well cooked, and I doubt many would be able to tell what you used. My mom used to make exquisite pastry cream with skim milk, and few if any were the wiser... (OTOH, my mom could have made cardboard taste delicious).

I totally agree with BakerRB. I've bought some wonderful cream from both a farmer (raw..and incredible) and from a high end grocery store in a glass bottle. The taste difference was subtle, but noticeable in the store bought and much more pronounced in the farmer's cream. I'm not sure the difference would be worth it in a pastry cream. I'd probably choose to spend more on the chocolate instead.

I like the idea of using a vanilla bean instead of vanilla extract. Pleasing to the eye and palate.

I think it's fairly subtle and not everyone can taste the difference (also I bet some people can tell a big difference and that I'm somewhere in the middle). I'd say not "substantial". The most difference would be if one is pasteurized (generally fresher tasting) and the other is ultra pasteurized (generally more cooked tasting). I'd most likely go with the less expensive since pastry cream is quite cooked. I have tried pricier creams in glass bottles and been pleased with the fresh taste, but to me the eggs and starch in pastry cream overpower the difference in cream and it isn't worth the higher cost. I'm not going to say there's no difference, but I don't think your end product will suffer. Maybe if also using vanilla bean instead of extract I'd spring for it just to do it; best foot forward and all that. As for where it matters, I pay more attention with vanilla ice cream, creme anglaise, a creamy dessert where there's a lower proportion of cooked egg than pastry cream has. Going to be good either way.