lluuk;
I was in your same position until I had the chance to hear all 3 through with my portable rig.
The Senns cover slightly more surface area of the ear than the Beyer's and as a result I think they seem to find a place and seal on your ears a bit more easily. The Beyer's could take a few iterations to set up, if you will. Fortunately, you can manipulate (bend) the headband to adjust the clamping pressure / isolation if needed. I honestly had no major problem here as both sets of cans fell into place for me fairly easily. The Beyer's had a bit too much clamping pressure out of the box , this took about 15min. to adjust and at this point are plug and play for me.
One thing I forgot to mention is that the Senns (Amperiors) have an additional level of termination between the cans and the 3.5mm plug to make way for a volume control/mic...I was not thrilled with cabling on the Senns. at all.
So, if you are looking for that "fun", less reference type of performance, I'm not sure what the Amperior's add in for the $200 over the HD25s. If you are looking for reference clarity and detail, with brilliant mids the Aperiors seem to fall short of the 1350s at a higher price, IMO. I think Senn could/should have done more than just replace plastic with colored aluminum when trying to sell an existing $150 headphone for $350...updated drivers perhaps?
I don't know what other gear you have, but I think the DT1350s or the HD25s would be a great option, then use the savings on a good quality portable amp/source setup to go with your new cans. Only your ears can tell and I highly suggest taking advantage of a liberal return policy so you can make your decision that much easier.
Good Lluuk!