You need to give more information about your code to give solid answers.
Coding for performance is a skill, and coding for Java performance is a very unusual skill. It can be done, but there is a lot of mis-information and opinion around.
$ for $, the difference that one CPU will have over the other is likely to be minimal (within 30%) unless you are using very specific extensions that have been optimized for one CPU. I wouldn't even begin to worry about such tiny performance gains if you have control over the code. Improving the code will win every time, and the CPU will not matter.
You'll probably get more impact from:
1. Rewrite (in a simple minded way) into C/C++, or do native code compilation [under 1,000%]
[This rules out applet behavior]
2. Apply a profiling tool (google, there are several), and rewriting the bits that account for most run time, [over 1,000% improvement, but often more]
3. Redesigning the algorithm used to be more efficient. [probably 10,000% improvement, especially if combined with 2].
These are typical numbers, YMMV, and may even be much bigger. Hardware differences are almost 'noise' compared to 2 or 3.
If performance matters do 3; if you are planning to run 15,000 times, there are likely plenty of ways to make things MUCH faster.
Of course, improving the software is more like hard work, and maybe less fun, than throwing money at the hardware