With a portable sitting on your belt, your body proximity to the antenna will detune the antenna and cause less signal to be received. When networks are planned, we usually figure on this loss to be an average of 10dB... quite significant. Personally, I'd be budgeting for 15dB.
However when you pull the portable off your belt and hold it up to your face, the antenna is a little more in the clear, that is not so close to your body and so the antenna performs better... still not optimum but definitely better than when on your belt.
The idea of the shoulder mic is to get the antenna away from your body and always have it somewhat out in the open. Again this arrangement isn't optimal, but its better than being on the belt of the user. However counting against this is the loss of the tiny feedline in the mic cord, pulling things back a few dB.
The main advantage of this arrangement is getting the speaker near the users ears, but then any shoulder-mic will do this of course. However a normal shoulder-mic will keep the radio on the belt of the user while they transmit and the detuning of the antenna effect comes into play then, causing a dramatic reduction in the usable range of the radio system.
In your situation, if you had your portable on your belt with a full 1/4 wave whip receiving, but off your belt and near your face to transmit, you'd have the optimum worst standby RX but optimum best TX.
If you replaced this with a stubby helical antenna on a shoulder-mic then you'd have slightly sub-optimum RX & TX due to the lower gain stubby, but being clear of your body. You could improve this by 3dB by using a full 1/4 wave instead of a stubby on the shoulder mic.
If you're looking for the absolute best RF performance, then get yourself a 1/2 wave end fed antenna and hold the radio in your hand. A 1/4 wave elevated feed works just as well (some say a little better but I'm not convinced of that) but is slightly longer in length and not as flexible, so the 1/2 wave end fed is more convenient in my view.
If you must get a shoulder-mic, then the best performance will be realised with a mic that has a short straight cord, not a curly cord type, and using a 1/4 wave antenna on the mic. It's a little less convenient than the stubby for sure, but the stubby antenna knocks off about 3 to 4dB of performance, and combined with the mic cord antenna feedline loss you're starting to loose quite a bit.
Even less physically convenient would be fitting the shoulder mic with a 1/2 wave end fed antenna... but it would give you about 3dB over the 1/4 wave, potentially even more in this situation.
In the end, which way you should go is determined by how you'll use the radio and what level of convenience you want (or inconvenience you're prepared to accept) versus the RF performance you need.
