「」、。
You can shrug it off with an 'oh, really?' or you can take it seriously — either way is fine.
Literal
"Oh really" [quotative-と] let-flow [nominalizer-の] [also-も] good, take-seriously [nominalizer-の] [also-も] good.
A 'take your pick' piece of advice. あっそ is a casual, slightly dismissive 'oh, really?' — a noncommittal acknowledgment that doesn't engage with the content. The pattern ~のもよし, ~のもよし ('doing X is fine, and doing Y is also fine') offers two alternatives without preference, using よし (the classical/written form of よい 'good') for a slightly literary flourish. 流す ('to let flow, let slide, brush off') is used here figuratively: あっそと流す = 'dismiss it with an 「oh really」,' 'let it pass without engaging.' 本気にする ('take seriously, take to heart') is the opposite — engaging fully with what was said.