Both of my recent Pentax telephoto deals have now arrived, so comparing the two will be interesting!
First came the Spiratone 300mm mirror lens. I have been curious about this type of lens but never won a bid until last week. At f/5.6 this one has a better chance of AF systems working with it, compared to the slightly slower f/6.3 Tokina. That lens is micro43 only, while this is conceivably film-friendly - and arrived PK adapted and stabilized on the K·50!
Today the Sigma 100-300 arrived. This is my fourth copy I believe so it's very familiar to me! AF speed is really fast, telephoto optical speed is dreadful at f/6.7 - this is why the Pentax 55-300 and this lens alternate in my bag. The other good side is price; the 55-300 is an excellent deal at $100ish, while the Sigma came for $9.99 plus shipping and handling (about $22 all told). The Pentax is a bit more crisp and contrasty for the most part, but really with minimal retouch the results are very nice with either lens.
I have a PK-m43 adapter coming, but perhaps a T-m43 would also be useful. That 600mme closeup should be fun to pull off in good light! In the meanwhile though, the 45-175 should arrive soon to take the mic43 system to greater lengths, even though the internal-zoom lens won't get any longer!