In my opinion you won't be able to tell the difference between either amp. I have a lowly Purifi Eigentact to drive my LS 50 Metas and find it perfectly transparent at 104 db Sinad, so I'm not trading up to either Topping.
My biggest caveat is the LA 90's reputation for unreliability. It's one of. if not the very most, unreliable product(s) Topping sells. Topping does offer some great stuff. I have a A70 Pro headphone amp, and could not be happier. I'm certain if either of those two amps are working properly, they will fulfill your every expectation, but I do have serious reservations about the LA 90, and suggest you read the thread on ASR before pulling the trigger, so you know what you're getting into. If you do decide to buy it, go with a retailer with an easy path for returning it, like Amazon.
My biggest caveat is the LA 90's reputation for unreliability. It's one of. if not the very most, unreliable product(s) Topping sells. Topping does offer some great stuff. I have a A70 Pro headphone amp, and could not be happier. I'm certain if either of those two amps are working properly, they will fulfill your every expectation, but I do have serious reservations about the LA 90, and suggest you read the thread on ASR before pulling the trigger, so you know what you're getting into. If you do decide to buy it, go with a retailer with an easy path for returning it, like Amazon.