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!
My first shots with it were surprisingly contrasty and pretty well focused. These lenses are often criticized for low contrast so that was a nice surprise. More tests are needed, and the 'donut bokeh' has yet to be featured in the few shots I've taken with it.
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!