Meh. I like both players but Seiya will be 32 after this year, gets injured a lot, has issues defensively. I think we can at least shop for a better long term bat and take our chances with him hitting FA. If he's willing to accept a 2-3 year extension I'd definitely consider it. Happ will also be 32 (I was today year's old when I realized their birthdays are 6 days apart!) and solid all around but not as good of a hitter as Seiya. If his bat declines much it might be rough over the next couple of years.
Again, I wouldn't be against either of them being back next year (or whenever they play baseball again after this season),but I don't think extending either is a no brainer either.