You need to let responders know your preferences as to volume, tone and price. Glen Schultz makes PVC whistles called water weasels that are IMHO excellent in the keys you seek (A, G). DO you play alone, in sessions, church, etc.?
I think the up-and-coming Serpent Music Low-D WMD (Whistle of Mass Destruction) can be fueled to perform as an IRWMD (Intermediate Range Whistle…). If something bigger is needed, I understand that Alba now makes a low B-flat, which might well qualify (with proper fueling and the addition of Whistle Fluid Biological Weaponry, DOD code name WhisFluBioWeap) as an Intercontinental, or ICWMD.
I have a Dixon A, and I see several people advocating the G on this site.
If you have to choose between the two keys, I’d say go for the G. The A is only just below a Bb (you can gat a Generation Bb for under $10). The G is that bit lower, it’ll be really nice and flutey, and sound great in a church. I don’t think it matters what key the music is written in, you can just transpose to suit the key you have.
I have a Dixon A and G and I love them both, but I find the A is the whistle I reach for to play probably 80% of the time. It isn’t a super loud whistle, true. But it’s just really easy to play. The G is also wonderful (and the reach is even OK with someone like me with small hands) but the bell note requires a little more concentration to get without squeaking. Playing the A is as easy as breathing!