There's no direct answer to that. It's your money so only you know if it feels like good value. Do you actually need to go to either place (I'm guessing not if it's a mileage run)?

It also depends on what dates will can travel. Longer flights tend to be better value for mileage and some great deals are available off season on typically "economy heavy" routes. The only time I've paid for A3 business class (if that's what you mean by J class) have been on ATH-DUB and ATH-LIS. Both are just about A3's longest routes and I paid around 160 Euro for DUB (4460 tier miles) and 180 for LIS (4453 tier miles) so DUB only cost me 3.58 cent per tier mile). ATH-TBS will get you 2855 miles which is costing you 7 cent per tier mile.

It really depends on so many things such as how badly you need the miles and by when.

Also look at connecting flights via ATH, depending on where you are based. I once flew DUB-ATH-TLS in economy for LESS than DUB-ATH would have cost - lots of extra miles!