If you know how to interpret the specs it's not to hard to figure out for yourself. Here's how to get a good approximation.
You need to know two basic things about a pair of headphones to estimate how loud it will be, the sensitivity and the impedance. You can find usually these specs on the manufacturer's website for almost any higher end headphone. The sensitivity is usually specified in decibels per milliwatt which is how loud it will be if fed a one milliwatt signal. Impedance is the alternating current counterpart to direct current resistance. Impedance varies with the frequency of the signal and is usually specified in ohms at 1Khz, and higher impedance will mean lower volume. Since decibels are a logarithmic scale and the relationship between impedance is roughly linear, the sensitivity counts for a lot more than the impedance.
Now let's look at specs for these IEMs.
SE530 (from HeadRoom)
Sensitivity: 119 dB/mW
Impedance @ 1kHz: 37 ohm
Westone 3 (from westone.com)
Sensitivity: 107 dB SPL
Impedance: 30 ohms
and for some perspective...
Beyerdynamic DT 880 (from HeadRoom)
Sensitivity: 96 dB
Impedance @ 1kHz: 250
As typical for IEM's bothe the Shures and the Westones have high sensitivity and low impedance compared to full size cans. The SE530s have slightly higher impedance but much higher sensitivity. Therefore the SE530s should be louder from just about any source. It would be a very sad amp that was ok with 30 ohms but not 37. There are all kinds of arcane reasons why this isn't a perfect way to figure out volumes, but for very large differences in sensitivity like this case, it's pretty much foolproof.
The SE530s are somewhat well known for their high sensitivity, their ability to be driven to very high volumes by just about anything, and to sound good from just about anything. Right now I'm listening to Vivaldi in a noisy office with my SE530s from my D2+ (which has quite powerful amp for a portable device) and 7 out of 50 is quite enough volume for listening all day. Either pair should be perfectly acceptable from just about any portable source though.