If fly economy with
Etihad (as they fly to Dublin).
I've never paid for a Business Class fare, but when I can I upgrade with points, or upgrade through the auction system and at check-in.
Once I then received an operational upgrade to First Class.
The difference in $ cost of a Business Class fare compared to economy is probably a factor of 4. Very hard to justify that, but with a mixture of points upgrades, and tolerating economy on the shorter leg, it becomes tolerable.
The difference in points for a redemption ticket (not an upgrade) depends on the airline, but I think with
Etihad is a factor of about 2. Compared to the $ cost that's an absolute bargain, but I'll likely never have the points to be in a position to do it (and have not done so). I maybe could if I collected credit cards, but my credit rating wouldn't support that - asset well-off, income not so well-off, and the credit ratings like income more than assets. My Amex credit limit is $2000 and they won't increase it!
The J experience is incomparable to Y on
Etihad. The inflight entertainment is the same (apart from the screen size), but everything else is different. More room, easier to move around, excellent food, including "on demand" rather than at fixed time. Sleep is not only possible but easy (though not comparable with sleep in a bed). No queueing for the toilet. Doing a "mixed class" trip means sometimes missing out on extras like lounges and priorities - transiting through Abu Dhabi, being able to have a shower and change into clean clothes on a clean body, makes an enormous difference to the trip. In J I can even travel in decent clothes (looking for that operational upgrade!) but change into comfortable clothes (trackies etc) on the plane. That's possible without being in J, but harder. Priority boarding and baggage collection is a curate's egg. I prefer late boarding to priority boarding, especially because
Etihad don't turn on the IFE until in the air, so there's no much to do but drink champagne if you board early - priority at security and immigration queues can make a big difference to the journey experience, when it's available.
Arriving relatively refreshed (at both ends) can also maximise use of your time.
Deep Vein Thrombosis is contributed to by many things - I probably sit still more in J than Y, because I'm comfortable in my seat, and always try to have an aisle seat in Y so I can get up and move around easily. But if DVT is more common in J, I suspect that it's more to do with the age profile of the people flying in J, i.e. pre-existing risk factors.
So in summary, if I had the points to do it I would definitely J. When it's a cash ticket, I'd go for Y and try to upgrade key parts of the journey.