Stephen Kitt has done what seems to me an excellent job of outlining features and when they were introduced. I'll take a slightly different tack, instead picking a single point in time, and pointing out differences between the two at that time.
I'm going to choose the 21164 as the Alpha to compare. It came out in January of 1995. It had a 266 MHz clock speed, and a quad-issue pipeline (i.e., could issue 4 instructions per clock). That was balanced between integer and floating point, so you could issue 2 integer instructions per clock and 2 floating point instructions per clock.
Intel's fastest processor at that time was the P54C Pentium. I believe at the time, it had a maximum clock speed of 75 MHz. It had dual pipelines, so it could issue (at best) two instructions per clock. The second pipeline was fairly restricted, and scheduling was static, so in a given clock cycle, the first instruction (almost) always went to the the first pipeline, and then the second instruction issued to the second pipeline if and only if it was one of the specific instructions that the second pipeline supported.
To get an idea of performance (well, okay, my aim was a bit more selfish: to try to justify buying an Alpha workstation) I did some simulations of running Alpha code for a program I had at the time. It averaged around 1.6 instructions per clock.
I had a Pentium at the time (a 66 MHz P5). The same code running on it ran at about 1.1 instructions per clock.
The Alpha instruction set was rather simpler, so you needed to execute more instructions to carry out a particular task with it than with the Pentium. If memory serves, this was about a 2:1 difference, but varied a fair amount.
So, at least at that point in time, the Alpha was effectively about 3 times the speed of the fastest Pentium.
I feel obliged to address one more point though. You said:
In the late 1990's, Alpha fizzled in the face of the HP/Intel
partnership pushing their Itanium 64-bit architecture.
In my opinion, this is basically wrong. It wasn't Alpha that fizzled. It was DEC that fizzled. Continuing from the performance comparison outlined above: my numbers were convincing enough that I eventually got permission to buy a DEC Alpha workstation, and got funds allocated for it.
So, I went through the DEC catalog, and picked out exactly the workstation I wanted. Then I contacted DEC. The first guy I talked to was very enthused right up until he heard the size of company I worked for - we were too small a company, so he couldn't sell me anything. He gave me somebody else to talk to. So I talked to them. They were very helpful until they heard what I wanted - they weren't allowed to sell that workstation.
This went on for over a month. I spent weeks calling different people at DEC, and an almost bewildering number of VARs and VADs and god only knows what else. On essentially every call, I was very clear about exactly what I wanted, and that I had funds available to buy exactly that, immediately. I also made clear that assuming this worked out, my boss and his boss were both probably going to buy similar machines soon (there was no way a mere peon like me was going to have the fastest machine in the company for very long!).
In the end, I simply had to give up. I had the money. I had permission to spend it. But no matter how hard I tried, I couldn't get DEC to take the money.
At least in my opinion, that's why the Alpha died. I don't claim to know sales particularly well, but I'm pretty sure an effective sales strategy does not include refusing to sell your product, even when you have a legitimate customer who's not only ready and willing, but in fact downright eager to buy your product.