Negotiations on mowers depends on a lot of different things. Can the dealer come down on there price the answer would be yes. What percentage you can get could also be dictated buy you as the customer and what future business you may bring him. Generally speaking the homeowner doesn't get to negotiate the price a whole lot. The RSD Msrp I believe is $3999 that's the Lowes and HD price so his quote or price of $3899 is already beat theirs.
You say I want to buy once and I don't want to buy another mower for 10-15 years, I personally would look for a good used commercial mower with low hours, better quality drives and engine. I don't believe you'll get get that kind of time out of either of those mowers.