Summary:
- Primecoin uses the Fermat primality test, which has some flaws.
- Primecoin numbers are not guarranteed to be prime
- A centralized checkpoint system is implemented inside Primecoin.
- Riecoin is capped to a fixed amount of coins (84M), but Primecoin has no limit.
- With Primecoin it impossible to estimate how secure the network is.
It looks like the flaws of Fermat's test a are more rare than I thought, but they are still there. If you wanted cryptographically secure primes, the Rabin-Miller tests done by Riecoin are overwhelmingly more than enough (according to NIST standards) while Fermat's test isn't.
Also:
- Most importantly: cunningham chains != prime constellations
- We are testing Hardy-Littlewood k-tuples conjecture for k=6. And it's not even known if prime sextuplets are infinite. I find this more interesting than Primecoin's scientific value.
- Primecoin's primes are limited to a maximum size while Riecoin's aren't. This may imply Primecoin's verification is faster but more ASIC friendly.
As for which one is better, I'll let you decide. Primecoin still has 30x Riecoin's market cap.
Of course I like mine and I'll defend it

Hopefully, open source pool software will be a great advantage that will spawn many pools. Will post news soon.