ben88 - both brands can be excellent choices, but you're comparing ( or trying to ) apple and oranges. Grain not withstanding, no two Castello's are the same. Each one is slightly different, even looking at groups of the same shape. Looking at a group of Peterson's, they are all the same. Very little variation in the bowl "feel". IMHO each Castello is a jewel on it's own.
Before you select a Peterson, ask the vendor to check for fills. Even in the higher smooth grades. I have a gorgeous Sherlock Holmes on my site, and was surprised to find a 1/4" putty fill on the side. You will not find a fill in a Castello.
IMHO one of the best "bang's for your buck" on the estate market are higher end smooth Castello's. Most are selling for around 30% of the current SRP. For in the around $300.00 mark, and less you can find upper end Collection, Occhio Di Pernice's, and on occasion even a Fiammata. These are spectacular and one of a kind. Some might consider a Fiammata a collection center piece.
You may also find some new smooth Castello's on E-Bay, offered by sellers in Italy that are priced only a small percentage above the current US SRP.
All the above is of course only my opinion and I am in no way swayed by the fact I collect Castello's.
Regards
Michael J. Glukler