Any prep that saves you time is the shiznit...
As far as between the two for Sheppard and ATP... you can't go wrong with either. The difference that I look at is the "Time = Money" factor.
You spend $75 for the sheppard software and roughly another $90 to take the test at a CATS or Lasergrade testing facility... so that's $165 dollars.
You spend $295 at ATP for the Test Prep which includes the actual test.
The Sheppard option is $130 cheaper... unless you factor in your time. How much is your time worth? I'll use roughly 2nd year FO pay of $35/hr. Even though I personally feel we're all worth more than that...
Anyway... take the average of 20 hours it takes to prepare with the Shepard software vs. the 10 hours for the ATP prep. Now take the difference of 10 hours and multiply it by what you feel your personal time is worth. Using $35/hr... the difference equates to it costing an additional $350 of your personal time to go the shepard route. Using what you personally feel your time is worth makes the difference even more... making ATP the cheaper option when it comes to "personal time".
Sheppard = $165 + $700 (20 hours avg. prep time @ $35/hr) = $865
ATP = $295 + $350 (10 hours prep @ $35/hr) = $645
Net difference: If you subscribe to the Time = Money theory... then ATP is $220 cheaper.
Either way though... they are both good options. If an ATP facility is not close by or at least 1 easy jumpseat distance from you... then I'd take the Sheppard route. If an ATP facility is close by... it's a no brainer IMO.
Bob