What flow rate should I select in a shower head for a home with low water pressure?
The 2.5GM shower head will provide you with better flow (but will cost more in water and water heating) - and could also cause lower pressure elsewhere while the shower is running.

Removing flow restrictors will likewise increase the flow of water and feel more powerful.

It is possible for lower flow heads to perform better then might be expected due to the venturi effect and mixing air with the water, but I expect that if you have low pressure this will not help you.   

If you are purchasing a new head in a bid to fix your low pressure issue, might I suggest removing the flow restrictor first - if that fixes your problems, go with a lower GPM head to save water (because the pressure will be there), otherwise get a booster pump.