am8117

Shopping for a new Shimano RD for a bike with 44-32-22 crakset, 8-speed setup but intending to have it upgradeable to 9-speed. The current cassette is 11-30 and NOT expecting 32+ for the future, but rather a road-like one.

There's 3 RDs I can find currently on sale with rather different specs:
1) Sora RD-R3000 GS [https://bike.shimano.com/en-EU/product/component/sora-r3000/RD-R3000-GS.html]
2) Alivio RD-T4000 SGS [https://bike.shimano.com/en-EU/product/component/alivio-t4000/RD-T4000.html]
3) Alivio RD-M3100 SGS [https://bike.shimano.com/en-EU/product/component/alivio-m3100/RD-M3100-SGS.html]

The shifters to be used are rapidfires SL-M315. [https://bike.shimano.com/en-EU/product/component/shimano/SL-M315-8R.html]

All of them are 9-speed, but all of them should be "1:2" pull ratios and thus work with the 8sp and in the future possibly 9sp shifter, correct?

Sora GS is out of spec for the crankset as it has max front difference 20T, however should work with 11-14 to 28-34 cassettes.
Trekking Alivio (SGS only) is meant for 11 only to 28-34 cassettes, however in the future I might want to use a cassette starting at 12 or 13.
Mountain Alivio (SGS only) is out of spec for current cassette as it is meant for 11-12 to big 32-36 cassettes and future cassettes of my choice might be tighter, not large cogs ones.

Obviously, the M3100 comes across as most "modern", and considering all three can be had for about the same price. Which one would you go for and why? Which of the out of spec situations are more of concern than others? Anyone has experience Sora RD with a M/T triple crankset?

aggiegrads

Why not M-591? mountain and road are compatible at 9 speeds and you are still ready for future upgrades.
I would not worry about being under the max cog rating for a derailleur. Turning the b-screw out should make that a non-issue.

HillRider

The 11T small rear cog spec for those derailleurs is a minimum, not a limit. Any of those will work fine with a 12T or even a 13T smallest cog.

Also, the maximum front tooth difference isn't a hard limit. I have two bikes with 105 FC-5703 triple cranks and FD-5703 front derailleurs. The derailleurs are rated for 20T maximum (50/39/30) but I have replaced the 30T granny chainring with a 26T giving me a tooth difference of 24T and they shift just fine.

hokiefyd 

The M3100's upper pulley wheel (the "guide pulley") has an axle that is coincident with the cage pivot, meaning its vertical position is constant and does not change with cage position. I think Shimano have done it this way to maintain that vertical guide pulley position when shifting between front chain rings, but the negative consequence of this is that pulley doesn't track with the cassette very well. In my experience, if you set the B-screw so that the guide pulley spacing is correct for the largest sprocket, it'll have a pretty large gap with the smaller sprockets (because it doesn't pivot upward as the cage relaxes). This results, in my experience, in sluggish and imprecise shifting in that region of the cassette. You'll notice that Shimano's Shadow derailleurs designed for 1x drivetrains have the guide pulley axle different from the cage pivot, as with a traditional derailleur.

I understand Shimano's intent with the guide pulley here, but I've never had problems tuning traditional derailleurs with 2x and 3x systems, and I far prefer the traditional derailleur's tracking of the guide pulley height with cage position. I'd get a Deore RD-M591 derailleur or similar and enjoy thousands of miles of smooth shifts. The RD-T4000 is also a very nice unit (I have both the M591 and the T4000 myself). The T4000's cage knuckle is resin whereas the M591's cage knuckle is all metal. Both seem very durable and I'd use either one with confidence.