Buy the KitchenAid 7 Speed Hand Mixer.

It costs more because it actually lasts, the Black+Decker is the cheap one that breaks the way you'd expect.

What was compared

1. Beaters that don't wear out on you
KitchenAid uses stainless steel Turbo Beater II, Black+Decker uses reinforced nylon. Reviewers of the MX600T report beaters splattering batter past speed 2 or 3 and ejecting on their own mid-mix.
2. No surprise speed changes while you're mixing
Multiple Black+Decker owners report the mixer speeding up on its own and beaters popping out without the eject button being pressed. That's batter on your counter and a mixer you can't trust one-handed.
3. Stands up to heavier bowls without tipping
Reviewers note the Black+Decker falls forward when resting on its back with any weight on the beaters, a design flaw that shows up every time you set it down mid-bake. No such complaint surfaces for the KitchenAid.
4. Finer control from folding to full whip
7 speeds against 5 means a real low-end for folding in chocolate chips and a true top-end for whipping egg whites, not just bigger jumps between settings. For actual baking tasks that range gets used, not wasted.
5. Yes it's $45 more. Still the smarter buy.
The Black+Decker is cheaper on the sticker, but a mixer with beaters that eject themselves and speeds that jump on their own isn't a deal, it's a return waiting to happen. The KitchenAid has sat on winner lists for years because it just keeps working.