We have been on both lines and have been on both sizes/class of ship on O. In fact, although we are Elite Plus on X, we consider ourselves "Celebrity Refugees".
We concur with the comments above:
Food is quite a bit better on O.
Entertainment and activities are quite a bit better on X.
Less crowded and more laid back on O and nearly dead after dinner.
Demographic runs older on O, but we seldom saw any kids or teens.
As to O life - the comment above, that said to search the O board for detailed explanations on how to handle O life, is spot on. Once you understand the options, it is easy to decide what is best for you. And O is not always more expensive for a standard balcony.
Disagree with the opinion that O air is never a good idea. O air is never a good idea for Biz but O features some good deals on PE and coach. Not always, but some times. O does charge deviation fees in some cases.
Some TAs offer cash rebate checks when you complete an O sailing. About 10% can be had. Not really any different than refundable OBC that can be had on X. Consortiums sometimes pay gratuities on O.
OTOH, we took a 7 day Caribbean on the Apex in January just to get back on a ship after 2+ years of not cruising. And we liked it well enough that we booked the Eastbound TA on Apex next April. If we enjoy that sailing, we will continue to consider X, but only the Edge class for us.
HTH/YMMV