What do I get more Value using the flagship to travel or a Amex for travel ?
In order to redeem it to travel purchases?
For redemption they are going to be the same if using a travel portal to redeem. The cards earn at different rates for different categories, though. As stated previously, it's possible to get far more value from Amex Membership Rewards by transferring to airline (and in some cases, hotel) partners.
My friend just applied for the Amex green card with the annual fee $150 but she regrets not doing the gold
Can you change after the approval for the gold cars or you have to wait?
I'm not sure what there is to regret about getting the Green now. Dining earns 3 MR, nearly as much as the 4 MR on Gold. Travel points earnings are better on Green. The only area where Gold is better is groceries.
And I would say that if straight cash is what somone is interested in, the BCP would be a better choice at 6% though it is capped @ $6K yearly.
OP, your friend can always use the card for a couple months to earn the SUB. Then just app for the Gold to pool more points. And if she doesn't like how the Green is structured by her anniversary date, just close it. The Gold's AF is $100 more and a different rewards line up.
So she really should compare the two to be sure which one suits need best.