I agree, out of the two the 780 ti is more comparable to the 290x and usually beats it. The 780 is more loose than win against the 290x and is more comparable to the 290. Normally id say go 780 route, since its a lot quieter and runs way cooler than the 290x, take a bit of performance hit but overall better experience.
But since your watercooling the card, you essentially eliminating the only big downfall of the 290x, so its really a no brainer (if costs are similar). Id honestly pay up to $40 for the 290x, if its anymore than id go 780.
Also, remember when buying both card and block. Make sure card design fits block design. Some cards are reference, some cards are classified, some cards are brand specific. So to make things easier...