I’ve asked a few people, but no one seems to know.
What is the difference (cutting/usage -wise) between a dremel 115 bit with curved blades vs dremel 115 with vertical blades? I’ve noticed Old Lumens and a few others use the straight blade version.
I have a new rotary tool, and I’m not even sure what I should be using these kind of bits for. I need a tutorial.
I found a few decent semi-tutorial type things mostly very basic info though. I’ll see if I can find links.
This one the “select the right tool” has a brief rundown of the various bits.
I tried out about 8 different bit when trying to widen the contact board step on a pill. The most effective for me was a very small version of the links posted in OP, and surprising to me, the sanding drum. I absolutely despised the grinding stones; they loaded up with aluminum and just wouldn’t work for me.
I use the straight blades for my stuff, because they keep seem to keep from grabbing as much as the angled blades do. I suppose it's just personal preference, as I have used them both. Most everything I bore out is hand held, with the bit spinning in the drill press and my hands holding the part. The straight bits seem to be easier for me, but they all will cut.
Also, I have found bits made of HSS, (High Speed Steel), work better than Carbide, for cutting aluminum and copper.
The curved blades allow for a smoother less vibration cut. With the straight blades, imagine the cutter laying on a flat piece of metal, then rotate it, the blade will be cutting only when the tip of the blade contacts the metal, the rest of the rotation is in between the blades and with no cutting action, it does this for each blade. It acts like a flat spotted tire, its always riding the high spots and making a small vibration every time a blade hits the work piece. In wood it wouldn’t be as noticeable.
If you lay the angled blade cutter down on a piece of flat metal, you can see that no matter where you rotate the cutter there is always a part of the curved blade touching the metal. Its constantly cutting with every slight turn of the cutter, in different locations in the cut with every slight turn. Less vibration and smoother cuts.
If you need a cutter that will last and cut steel and aluminum, I have found nothing better than carbide burr, they will cut through aluminum like butter. This is one of those things where you get what you pay for, just make sure it uses a 1/8” shank.
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http://www.traditionalwoodworker.com/Cylinder-Radius-DC-Carbide-Burr-1_4-Head-1_8-Shank/productinfo/63-23202/
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The double cut flute will tear through aluminum and metal. I couldn’t be sure if the one linked is quality or not, it says its made in Germany so it should be. A USA made brand should also be of good quality. The one I have, I bought locally at a large specialized tool store. Mine was made in the USA and has lasted going on 8 years. Cuts as good as the day I purchased it. Just look out for the tiny metal shards it produce’s when cutting metal, they can end up in all sorts of different places under the skin. 
Ah, I didn’t even notice the blades going the opposite angle like in these vs the ‘single’ cutting blades linked in op.
When you are talking about tools like drill bits, end mills, cutters, machinist type tools, you usually get what you pay for. Not saying there isn’t some good deals out there on some quality tools, there just pretty rare. Just my experiences. 
I have a small set of these milling tools from DX and they are very good for aluminum and copper work.
I planned to order a dremel bit but after testing these with no problems I have no need for that…
I usually use the biggest one because it mills very fast
I also ordered a bunch of polishing tools, which are handy for pill polishing because the ones I had earlier had a screw on top…
I bought a Ryobi multitool that came with a flexible drive and a clamp-on stand, clamps onto table etc, for what I use it for thgis should be your first accessory purchase.
I buy pretty much non-Dremmel tooling as Dremmel are expensive for the quality you get, or don’t get. 
e-bay and Chinese tooling are a great combination e.g. 30 diamond coated burrs for £4.99, about $8, they last well even on knife blades that have been heat treated (damn you serrations, begone), a very hard use, they last about 10 mins before starting to shed the coating but at the price I’m happy enough.
For drills I try to stick to the collet sizes of 1.6mm and 3.2mm, I have a Dremmel drill chuck but it is rubbish at £9 $14, I bought a full sized keyless chuck for £3$5 recently, actually it is rubbish at any price, not securing the drill bits centrally.
moderator007, I’m going to have to try this burr , that and the square one would cover pretty much all I need.
Ryobi multitool , mine came with carrying case and stand plus loads more accessories than shown here - B&Q £49.99