The silhouette of both phones is virtually identical (the famous rounded rectangle) but the dimensions differ. Most crucially, the Samsung Galaxy Alpha is only 6.7mm thick, more than 30% thinner than the 10.6mm HTC One mini 2.
The Galaxy Alpha is half a centimeter shorter too, despite having a bigger screen. HTC make this tradeoff to make room for the two front-facing speakers. The Alpha is lighter too, 115g vs. 137g, though the difference is small.
What makes a bigger difference for in-hand feel (and looks) is the back panel. The HTC One mini 2 has a sealed metal unibody and cards are put in trays that slot into the sides. The Samsung Galaxy Alpha has a removable plastic back cover so you can access the battery and SIM slot.
The premium feel of metal is a huge boon for the HTC phone while the Alpha doesn't even use the best plastic from Samsung. The dotted pattern is too small on this one so it doesn't quite look like perforated leather and the material feels smoother (more like plastic).
The exposed aluminum from the frame of the Alpha does improve things significantly compared to an all-plastic Galaxy. Still, a glass back (a la iPhone 4/4s or Xperia Z) or a matte polycarbonate (like Lumia 925/930) would have been a better choice.
The HTC One mini 2 has a rounded back which hides some of its thickness. The sharper edges of the Samsung Galaxy Alpha tend to poke into your wrist a bit, enough to remind you that it's made of hard metal but also enough to be slightly less comfortable than the smooth, round back of the One mini 2.
Both phones have their microUSB 2.0 ports on the bottom and their 3.5mm audio jacks on top. HTC has put the Power key also on top so it's slightly harder to reach on the taller phone.
On the flip side, the on-screen buttons are further up the phone so they are easier to reach. On-screen buttons, however, mean the One mini 2 has less screen real estate left for apps.
The hardware home key on the Galaxy Alpha is very near the bottom and requires a more flexible thumb. It houses a fingerprint scanner of the swipe kind and using it is a learned skill - you have to run the entire top digit of your thumb against the key and the scanner can be sensitive to the angle at which of you're doing it.
Speaker placement is important. The BoomSound speakers on the One mini 2 are on its front, while the single speaker on the Galaxy Alpha is at the bottom. Neither will get muffled when you put the phone on a table but the HTC is better at directing sound outward.
The sides of the devices feature a volume rocker. The one on the Samsung is a bit too thin for our liking, while the one on the HTC is too flush with the sides. Overall neither is the most ergonomic to use.
The HTC One mini 2 has card slots on the left and right and you'll need a tool to open them. The SIM slot on the Galaxy Alpha are accessible without any tools but you'll need to remove the battery.
We come back around to the back of each device. We covered the differences in material but there's more to talk about. The Galaxy Alpha may be thinner but its camera module protrudes from the back, making it more vulnerable to damage. Next to it is the LED flash and the heart rate monitor (check the camera lens for finger smudges after measuring your heart rate).
The HTC One mini 2 camera is flush with the back and also features a single LED flash. The secondary microphone is here too, while the Galaxy Alpha places its two mics on the top and bottom.
Winner: HTC One mini 2. There are plenty of half metal/half not phones out there from Apple, Sony, Nokia and others but metal unibodies are rare. The One mini 2 is certainly not the most compact 4.5" phone out there but it's still a joy to hold - and has stereo speakers to show for it.
The Galaxy Alpha is one of the best built phones by Samsung (the metal-clad Waves as good as forgotten). It's impressively thin and light too and doesn't add much bulk around its 4.7" screen. Still, that so-so plastic on the back doesn't do it any favors.