Both Yachtsmen and Jiko are "nice". They are very different. Jiko has a much more upscale atmosphere. Food is good at both - it depends on if you are looking for a "Steakhouse" or an upscale restaurant. Yachtsmen is a wonderful steakhouse - we've always been very pleased there. (But then again we're the type of people that always order the filet at Jiko!)
If it were my birthday dinner, I would let my other plans help me decide. From Jiko you can walk around the AKL and look at the animals but from Yachtsmen you can hang around the Boardwalk area.
It's a hard call - they're both very good!