The 4 stroke will be heavier, quieter, more efficient, use less oil, not require mixing, more complicated to fix, newer, less powerful and the list goes on and on.
You'll have to decide for yourself which pros/cons make a difference to you.
If it were me, I'd pay the $1,500 in a hearbeat to get a 15 year newer 4-stroke motor (assuming its in good shape).
Your motor is worth somewhere between $1,000 and $1,500 if you sell it on Craigslist.
That 2005 Yamaha might sell for somewhere between $3k and $4k I'd guess.
Sounds like it could be a good deal.