The main difference between a cash bid and unit bid is unit bids can immediately level the playing field (if the game is deemed unbalanced or it plays that way in your play group). The general idea behind a bid is to give both sides an equal chance to win while changing the fewest possible things.
With previous games (Classic and Revised) the setup needed slight altering and cash just can’t solve the problem. Take a G1 Egy attack in Revised. With no bid you attack, maybe win, but are easily countered on UK1 and Afr is lost for the entire game or not a real issue for the Allies and the Axis will lose. It doesn’t matter if Germany has 12, 15, 18 extra ipcs to spend on units on G1 b/c those units won’t see the Russian front until G4. By that time UK and US are landing heavily in Nor/Kar or threatening WE/Ger with a direct assualt. The issue here is how do you help boost Ger’s income long term for the mid game. The answer is they need to be able to blitz Afr, thus they have to hold Egy on G1 and prevent the UK counter, so a bidded unit becomes necessary. Simply placing 1-2 more inf on Lib is much better than the equivalent 12-15 ipcs needed in cash. FIDA bids (split bids between land and cash) are typically double unit bids so if you bid 8-9 in Revised, you should be bidding 15-18 for FIDA and I’m assuming a straight cash bid would be even higher, I’d figure at least 21+.
Now for AA50 there’s still a debate on side advantages, but again it all depends on what eventually becomes the standard G1 and J1 open. Right now Ger attacks the 3 Russian Ter, Egy, Sz 12, Sz 6, and probably Sz 2, and Japan attacks 3-4 chi, kwa, pearl, sz 35, sz 56, phil, and takes the empty islands. But as play on both sides tightens up if it is ever determined that one of these early crucial battles is vastly more important than the others then you’d need a unit bid to solve the problem.
For Example, I believe Egy could be this battle in AA50 as well. I think leaving the UK with an arm/ftr or ftr gives the Allies an adv to win. However, leaving the UK with a BB, DD, and 2 trns to start in the Atlantic isn’t that great either. Thus both should probably be attacked but in order to do both you have to take some risks (either no bom to Egy or sub(s) + ftr(s) to Sz 2). If you win great, but if not, you could be in an even bigger hole. So to remedy this why not just bid another inf to Lib and make Egy a little safer? Again, if this battle is viewed as the most important of round 1 then giving Ger, Ita, or Jap cash won’t help in winning the battle. On the flip side if you think the Axis have the Adv in AA50, there is still going to be 1 key battle where you hope your Allied defense holds. That will likely be Egy, a Russian Ter, or perhaps one of the naval engagements. For simplicity assume the Allies feel Egy is the key battle. Again giving cash will not help if Ger always wins in Egy, or if you think BST is the key batte b/c then Ger always takes Kar in Rd 2 and smashes Russia in Rd 4 or 5, why not just bid a couple inf there (or Kar) so Russia can maybe counter BST and hold Kar. In the last scenerio giving Russia Cash will not help counter on R1 and may not even help in R2. Thus you still lose the strategic goal and position for the cost a few inf. All that means is if the Axis gain an Eco Adv it might take them an extra turn or two to crush Moscow. In this case the cash bid never gave you an oppotunity to get the game to a 50/50 proposition.