Go for 38/357... they're identical in all respects apart from case length, so a set of dies for 357 will also do 38.... you can then pick and choose which cases to buy, or scrounge, and use them as your starting point.

Something to consider is whether you use 38 cases in a firearm capable of using 357... there is a 1/10 inch difference between the 2 (so you can't load a high pressure 357 round in a 38 revolver, but you can load 38 in a 357 firearm).

If you only use 38 cases, that spare 1/10 of an inch in the chamber will get filled with carbon and lead, so if you eventually want to load some 357 it will be tricky..

Personally, i've only ever used 357 cases for that very reason... you can still load light target loads (3 to 4 gns of a fast pistol powder) in 357 cases.

HTH