Do whatever feels right to you. What's the worst thing that can happen? Everyone leaves your campaign? 
I think, in general, once a PC *has* a special magic weapon, then it's very hard to upgrade it. If they've picked up a flametongue, the only weapon that is likely to replace it is a vorpal sword.
Personally, I'd be looking for little quirks, additional small powers, and/or intelligence to add to a weapon. If the +0 giant slayer also detect giants, then it's something that can be worth giving up the bonus for.
Cheers!

I think, in general, once a PC *has* a special magic weapon, then it's very hard to upgrade it. If they've picked up a flametongue, the only weapon that is likely to replace it is a vorpal sword.
Personally, I'd be looking for little quirks, additional small powers, and/or intelligence to add to a weapon. If the +0 giant slayer also detect giants, then it's something that can be worth giving up the bonus for.
Cheers!