All part of the fun. If I can spend just a little more and get a better serial number I do because I think it's easier to sell and would typically go for more. (I had a 1xxx Giannis SS yesterday that I listed and it sold at $490 even though the current low was $450). For the lower end guys I'm not sure it matter much but still think if you will be dumping them right after the challenge a lower serial will stand out.
In the Cool Cats... the AD was way cheaper to buy after the fact. But you do have a slim chance of pulling a really good serial number which would not be the case if you buy the challenge reward on the market afterwards.
My quick take:
RS stars are risky since they are rookies
Durant SS is likely not worth it unless you pull a great serial number, the number of completions stays below 3K and/or TS opens the Reg gates over the next two weeks and the new people drive demand up similar to what we saw in late Feb.
Lebron SS was worth it at yesterday prices but is now borderline unless you already hold Luka, Steph, etc. What I'm trying to wrap my head around is whether I should sell Luka, Giannis and others I'm holding and just buy LeBron S2 moments as those might go up more over the next month than the SS moments and EV of the LeBron challenge.