+1 for:
1. Buy from known vendors such as DigiKey, Mouser, Newark, ... to be sure you've a low chance of getting a fake.
2. Avoid water clean flux formulations. Too big a hassle for a hobbyist IMHO (must be cleaned off fairly quickly, and thoroughly, or it will cause corrosion).
As per Rosin or No Clean, that's up to you. As mentioned, though Rosin based (R, RMA, RA) may have more solids, it can be cleaned off fairly easily with IPA or good quality flux cleaner. Another member uses xylene (BTW, it mixes with IPA, though I've never tried it).
If you're not sure, try both and see what happens (IIRC, you already have a no-clean based roll of Kester solder). ***But if you only intend to get one type right now, you'd do better to match the flux pen formulation with the solder you currently have (prevents chemical incompatibilities).***