I'm part of the Forest Practices Group (FPG) of volunteers at the local big park. We have about 1,800 acres of timber to manage for trails (keeping open) and hazard trees (sick, possible falling on camp sites). We have saws from limbers and climbing saws to 2-man units for felling.
Rule #1 - cubic inches (cc's) count. Torque always pays off.
Rule #2 - long bars do not hurt anything on small stuff, they can save your back from bending down. But, it's always better to run skip-chain on long bars. The extra distance between cutters allows for better/faster chip removal so the cutters are working more efficiently.
Rule #3 - never skimp on oil. Big motors under load generate a lot of heat. Good oil and plenty of it will keep that investment running a long time. No motor ever died from to much oil. But there plenty that died fro to little...
Rule #4 -
NEVER lean out that motor. Always tune slightly on the rich side. It's OK if the motor is 4-stroking before it touches wood. As long as it cleans up and pulls hard 2-stroking under load, it's fine. It'll make more torque and be less sensitive to varying loads and hard spots, and keep cutting. It'll never fry a piston and you'll be happier as a owner
Rule #5 - Have plenty of plastic wedges handy. Long bars pinch easily in fat threes. Once you get into the stem, start pounding wedges behind the bar. Keep that cut open and working. Wedges are cheap. Powerheads, not so much.
We have had trees "Barber Pole" on a cut and rip the power head right off the bar. Plan your cuts carefully. If a bar gets seriously pinched, stop and remove the powerhead. Finish with another saw from the other side. Worst will be a bent bar.
In the case of your two selections, they each have strengths and weaknesses.
The side chain adjuster is not as hardy as the screw adjuster. I will not own a side adjuster, but that's just me. Good chain, well oiled on a good bar will not stretch much in a hour of running. You need to tension when you fill the oil tank. No biggee. You need a break anyway.
But, the 390 has adjustable oiler for the chain. And that is a biggee. Weet wood with spring sap rising, not a lot of oil needed. Downed dead or snag wood that is dry, you can't oil enough. We run Stihl or Oregon chain with the oiling holes in the drive links. Long bars can starve for chain oil. Being able to add more chain oil is good thing
So it comes down to what you cut mostly?
If semi-dry firewood and sick snag trees, I'd take the 390 and bite the bullet on the side adjuster. I'll take oil over nuts and bolts.
If bigger standing trees, I'd take the 395 with the long bar, skip chain, and I'd mix summer bar oil and ATF (75/25) and occasionally wet the bar with a brush of oil from my slop can if going into dry wood ...