I've experience only with the aluminum minis. I got one BEFORE I got an N-Joy Pure Plug medium (I should have bought the large). While lying on the couch one night, my ass very nearly ate the damn thing! Don't go there.
All the plugs you have selected are made of safe materials, and some are relatively inexpensive, so I would say they are definitely worth the extra cost over a less expensive plug made out of less safe material.
That being said, is the stainless steel worth the extra cost over the aluminum, silicone and glass? Maybe not, but if staying in place is factored in, one of the pure plugs might be worth it.
I've worn the Luxotiq Glass for extended periods, and the portion inside my body was very comfortable. The only difficulty was the base - if you're sitting for long periods it can irritate because it doesn't flex like silicone. It's fine if you're standing up, though.
Forgot to talk about Ringo. It stays in pretty well, but especially when you're new to it it's hard to shake the feeling that it's going to fall out. It's a big plug, but for its size it's easy to get in.
The only other thing about it is the base. It's a bit more noticeable than other bases. For long term wear I really like the base on the A-bomb, since it stays out of trouble.
The only one of these that I've tried and had problems with it coming out is the Flirt. Nice plug, feels good going in, but I always have to have a hand ready just in case. The other ones I haven't tried. My only concern with the Pure Plug is how well it would work if you're sitting down.
I love the feeling of stainless steel for an extended ware butt plug. The medium feels great sitting, walking and during sex. My b/f enjoys waring it as well. I probably should have gotten the large and probably will some day.
Which material is the best for a plug? Silicone, glass, metal, other?
Glass or Metal are my favorite.
For extended period of time get the Pure Plug 2.0
For a cheap glass plug which is probably my favorite out of all the ones I have tired, you need to look for the Don Wand Clear Plug.
EF doesn't sell it here (I wish they sold more DonWand products because their prices cannot be beat for the glass toys) but you can find it over at amazon.com
I'm not a fan of non-pliable materials like glass and metal for anal use, but it is definitely a matter of preference here. Keep in mind, both glass and metal can be very good for temperature play, so if that's something you're interested in, it could be a big part of the decision.
I have the encore and will eventually write a review but I don't recommend it for long term wear. It sort of leans inside my recum to the right and pins down a nerve inside really painfully, plus it slips out.
Depends on what you want to do in terms of wear and sensations. Some plugs are better for long-term wear (long necks, comfortable bases), others are better for active play (the Alumina is definitely in this category). The sensations are different, too, depending on whether it's a big plug, prostate plug, etc.
It's a big plug, but if you want a big plug for long-term wear the A-Bomb is great. Very stimulating if you're a man (can't speak about women, but I imagine it would still be pretty good). The base is comfortable, the bulge is right against the prostate, and it is very secure.
The Ringo and the Pure Plug 2.0 are probably going to be the most difficult to insert since they are the largest. If you're looking for a stretch though...
The rest of them might be pretty similar for this category.