Kitchen Aid 600 has had all metal gear boxes for years. They're good machines, and are always available for big discounts. I think the best reasons to choose a KA over its closest competitors are price and attachments. That is, if you already have a few KA attachments you won't want to abandon them, and/or if you have your heart set on and absolutely cannot live with particular KA attachments.
We've had one for about two years now we got to replace an older 6qt KA which died from stripped gears, and which itself was bought to replace an old KA from the late eighties which went to my ex as part of her part of the community property and is still going strong for my daughter. I like the new one just fine, and so far -- knock wood -- no problems. Since the last machine broke, I got in the habit of doing bread mixing and kneading by hand and continue to do so because I enjoy it. In any case, my expensive collection of KA attachments: grinder/sausage stuffer, grater set, pasta rollers, etc., fits.
The Hobart is very nice, takes KA attachments and has certainly proven its durability over the years, but considering its price and limited capacity, loyalty to it may not make as much sense as it did many years ago. I just checked ebay and the current asking price for a used N50 in good shape was $789. They're listed for around $2200 new, but I think you can get one for less if you look around.
FWIW, I know someone who did his own rebuild on a broken N50 and found the process relatively simple and inexpensive. I wouldn't take on the challenge myself, but you may like fooling around with appliances.
If you make a lot of bread or cookies, are willing to spend the big bucks, want something heavy duty, and want true two and three loaf capacity, the top-down gear drive machines like the KA, Hobart, Cuisinart, Viking, etc., have been somewhat superseded by bottom-up belt drive machines like the Electrolux and Bosch.
The top of the heap is the Electrolux made Verona / Magic Mill Assistent DLX. They're by far the best stand mixers for bread baking, cookie dough and other heavy duty tasks; have 7.4qt capacities, do all the light duty stuff extremely well (although they look pretty odd doing it), and have a good range of attachments (and speaking of looking odd, have to be stood on their side to operate the attachments). Odd ducks, but they work extremely well and last a long time. The price ranges from just under $700 (Pleasant Hill) for white, and just under $800 for most other colors.
I don't know how meaningful it is, but... if my wife hadn't purchased our current KA, I would have bought an Assistent and to hell with the old KA attachments.
If you're not going to be doing that much heavy duty mixing, go for the KitchenAid 600. Get it from someplace like BB&B or SLT which is better than good with guarantees and get an extended warranty going beyond the already very good standard "two year 'hassle free' guarantee," so you'll have some extra piece of mind.
Some other machines to consider are the Hamilton Beach CPM 700, the 7qt Cuisinart and the 7qt Viking. To my mind -- based on reviews and other online information -- they're all more or less fungible as mixers with the new 7qt KA.
Did you find this more helpful than confusing?