Better lounge access (Amex) and also milage rates (Everymile).
Fair few benefits from Amex.. but might not be worth the same for everyone and depends on which Amex u have. Explorer gives you some lounge access and Platinum gives you some more access ...
Everymile also has a few free plaza premium...
Everymile conversion rate at the moment ..
I am no benefits hacker ... So do your own research..
At the base level, AMEX Explorer and HSBC EveryMile are equivalent for standard cash back (1% of regular spending), and Asia Miles redemption ($5 spending per Asia Mile / or $0.05 RewardCash per mile depending how you prefer to calculate).
Each will have occasional redemption promotions: such as the 1 RC = 22 Miles above, but that is not the norm.
However earning the RC (or AMEX points) do vary. EveryMile is great if you buy petrol a lot or throw huge parties at coffee shops (effective 2.5% cash back instead of 1%) - but with the AMEX Explorer you'll get effectively 1.25% cash back for foreign currency spending, or 1.67% for spending on a range of airlines & online booking site (hotels.com, trip.com, Agoda etc...). Not as good as HSBC's 2.5%, but different categories: you'll never spend the same amount in Starbucks!
If you're not fixated on Asia Miles redemption, AMEX have frequent bonus redemption offers: e.g. 25% bonus value if redeeming from Apple, CX, Hotels.com etc... sometimes even higher. Last Christmas they had a doubled redemption value promo: I used $2,000 worth of points to knock off $4,000 from a MacBook purchase!
On the other hand, HSBC often have earning bonuses: keep an eye out for promotions in Reward+ App. At the moment there's a $600 RC bonus for $10,000 accumulated spending for example... applies to a range of HSBC cards, but makes sense to earn using Every Mile if you plan to convert to Asia Miles afterwards.