It is heavier than the 300/350 and quite a bit bigger, I mean a lot bigger, especially with the extension table in place. To my knowledge there isn't even a case that it will fit into because of the size of the carriage arm. Right now mine is sitting on two vintage machine cabinets set back to back.
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I have no clue whether or not a dealer will take your 300 for a trade in. The SewItsForSale Yahoo group is a great place to sell your machine though.
Deb, I've seen them go for $1999 to $2399. One thing to think about before you trade in the 300/350 though - If you use the tiny hoop (the optional free arm hoop) a lot, keep your machine. The 500 as far as I know isn't going to have a hoop that small. while I love the larger hoops on the 500, I'm gonna keep my 350 because I use that tiny hoop quite a bit. My 200 has both the 5.5 and the 2 inch hoop and I used both of them equally as often. That machine will either be sold or given to one of my girls.
Cari