Ace Hardware lists this mower for $630 but gives $30 discount for Ace Rewards members (free signup), which brings the price down to $600 (before tax), which is $50 cheaper than Home Depot $650 before tax list price.
This is the only Ego model where I found such price discrepancy.
The difference between this and Ego LM2102SP: LM2135SP is the "select cut" model which is 2-blade one - two blades inside the mower, one of those is switchable between mulching and bagging. So, it should have a better cut at the expense of slower speeds, but LM2135SP tries to compensate for that with brushless motor (not in LM2102SP) which should be tad more efficient and last longer.
FWIW LM2135SP is Wirecutter's pick [nytimes.com]
as of now for battery-powered mowers. It's also among the recommended ones by Consumer Reports', although CR more or less recommends all 3 Ego's self-propelled and neither CR or Wirecutter tested Makita's newest models yet (their 21" self-propelled equivalent XML08PT1 looks like a tank but it costs $850).