I've had mine for almost 2 months now & here's the path I followed.
dse901 with plenty of backup attys
510 with 15 or so attys
For me I went with the 901 version to use all the extra attys I had but quickly figured out that my 510 was going to be in my hand 75% of the time, only hitting the prodigy during breaks at work, lunch, at home..etc etc.
So instead of stocking up on 2 styles of attys, I'm focusing on just 510's at this time.
As for flavor I think it's a personal experience, just like juice flavors. I've heard people say an 801 atty on a prodigy is the best, others say a 510 yada yada...
Here's my take. You're cooking juice at 5v (stock) & from my experience, the taste from both my 510 & 901 attys/adapters are great for the first hit or 3, then tapers off quickly. Almost as if the flavor is cooked off before all the juice is (I drip)
Now if you wanted to get the switch without the resistor, that'd let you run at either 3.7v or 6v depending on what batteries you choose. I definitely think at 3.7v you'll get longer flavor & of course longer battery life. Thats up to you.
As I'm sure you know if you've been in the PS forum, Steve is coming out with the Protege`. Smaller than the prodigy, stock 3.7v & IMO, would be great with an 801 atty.
Basically if I were in your shoes, I'd go for the 510 adapter seeing how you're already using those attys & the stock 5v switch that comes with it. And make sure you get an extra few batteries. Even though most say they last 4+ hours & yes, I believe they do, I can notice a difference in throat hit after about 2-3 hours.
If funds aren't a problem, get the 3.7v switch & batteries as well...it's your call.
Rob