Christelrice, at the risk of being redundant and in my feeble attempt to clarify, Feadoggie is saying what we discuss constantly around here. Whistles are a personal choice. There are several vendors who tweak whistles, some of whom regularly contribute to C&F. But each "tweaker" (gulp
) has their own scheme, just like each maker tries to put their personal stamp on their instruments. It's not what we prefer but what you prefer that makes a difference.
I'll give an example. Cillian O'briain makes a uniquely tweaked instrument that I believe is based on a Feadog. Many consider it the finest traditional (whatever that means to them) sounding instrument. Jerry Freeman tweaks lots of instruments. None of them sound like Cillian's do but Jerry has a huge following, likely much larger than Cillians, however I have no data to back that up. Tommy Dion tweaks whistles and his are different from these other two. I forget who makes them but there is the "shush" whistle that is quieter than the rest.
I don't have a Shush but I have instruments from the other three. All are good to excellent whistles. None are "bad". Some sound better on certain tunes than others IMHO.
As to your question, when I'm playing tweaked whistles I go back and forth between Jerry's Bluebird Eb and Mellow Dog D. I find the Bluebird D just slightly more stable in the low end but I find the Mellow Dog to be better balanced throughout. Either of them will satisfy a basic whistle need. But so will the O'briain if you're looking for a "pure drop" tone. Tommy Dion's work on the Gen Bb is really good too as is Jerry's but that's a really good whistle without tweaking.
I doubt you'll find much to quarrel with on any of these instruments. But, as Feadoggie was stressing, it's not always about tweaking. It's about getting the characteristics that are personal to you. This weekend, I was out with my niece who wanted a good whistle to play but wanted to buy her own and wouldn't let me give her one of mine (kids!!!). She plays a little bit so I took out my four roll cases and let her try all the D's I had. She wanted an inexpensive one so she bypassed all my hand-mades and picked a standard Feadog Pro over all the rest, including those mentioned above. I talked her into taking a Jerry Gen along with her since I had two, and now, two days later, she can't decide which she likes best. Good think I didn't push her toward the Walton's Little Black since she's a huge Corrs fan but, didn't know about Andrea's connection to these.
Just pick several of one or a few of each and have your students play them till they fall apart. Jerry's offer is likely to be generous if I know him well and I think I do.