Do that search, and you'll have alot of reading on differences in construction, case materials and power-to-weight ratio.
These two saws really aren't comparable. Either in hp, weight or construction, though both will handle firewood and farm tasks easily. The 260 will be a gem to use due to it's weight, but the 390's hp will help it in longer bar applications, but if you're gonna be working timber or asking the saw to work for you on that level consistently, consider the 361. Same hp on paper, but a wider power band, especially opened up, better anti-vibe, less weight, and professional grade construction.
But if you end up with the 390, or 290 or 310 for that matter, don't base your entire impression on Stihl's line on those saws alone. Get a 44 or a 66 in your hands before you make your decision.
Jeff