I havent contributed to BOINC specifically to make GRC or Sparc, I contributed for the sake of contributing to science. I will probably continue to contribute to boinc as my primary goal and claim the value that is generated from that. I think that that is the majority consensus to most GRC miners.
Having said GRC has inherently more value as it is an actual block chain and Sparc is seemingly just a marketplace for compute power (or atleast it wants to be eventually). I dont really like that as it has the potential to take away compute from Boinc to those that are willing to pay for it on Sparc.
We will have to see if Sparc takes off, at the moment though there is no disadvantage to running both and I will probably just hold onto the tokens I accumulate while contributing to Boinc