I prefer blackstone. I bought 100 prepaid kits before the price went up. Change the oil and send it off pretty much every 1-1.5 months at 50-60 hours on the oil. I use their pump and pump it through the dipstick hole. The whole system is just clean and easy.
I also like that I can log into the site and have all the previous results for each airplane. I like that I can ask for things to be included in the notes section on the report. So if you have done some work or are trying something new it can be noted right on the results to view later and keep things straight.
Grab a kit from the box, cut a few feet of 1/4 ice maker line and stick it in the pump, screw the bottle on to the pump, pull up the sample, cap and drop in the prepaid envelope and done.