Korean language can be devided into two languages according to the listener.
- informal form (for friends/younger people), and formal form (for stranger/older people)
both 'saranghae' and 'saranghanda' are informal form.
I think it's not easy to explain the difference between these two sentences for koreans though
'saranghanda' sounds a little more masculin, strong and stiff.
and 'saranghae' sounds a little more friendly and usual.
it's a matter of MOOD. just one different phoneme at the end can change the mood of saying.