Did you try Sears? I recommended them to another poster recently, based on my own excellent experience with them. Short version: I looked online then called them for a replacement. They sent an excellent local mechanical contractor (licensed plumbers and electricians). They came right out. They were meticulous and excellent. They put in a drain pan, which I didn't have and which was important in this above-the-garage apartment. They would have put in an expansion tank if I needed it. It was a great experience, at a very reasonable price for the service I received (like $235 installation plus about $100 for the drain pan).
Unfortunately, it didn't work out for the person I recommended Sears to, but their response was telling. Come Monday morning (they don't work on weekends), she called before noon and they sent on the same day a licensed plumber on contract to them. (They don't send Sears repair techs; they hire a local licensed plumber to do the job.) They add $35 to the cost in her jurisdiction to cover the cost of a permit for the job. She was in a mobile home, and Code required a Sealed-Combustion-Chamber-Direct-Vent mobile-home certified water heater, which Sears doesn't sell (it's kind of a specialized item). Although two Rooter companies had offered to put in a water heater from Home Depot (which also doesn't carry the right kind locally) at an insanely-high price, the Sears plumber just told her, "Sorry, we can't do it because it's not legal." Sucked for her, but it was reassuring to me that they insisted on doing it right.
Bradford-White is an excellent brand, and recommended by some of the pros who post here. However, other pros here have observed that the Sears product is made by State (an A.O. Smith division), and is a good product that they will stand behind. One of our moderators indicated that water-heater longevity is primarily the result of factors other than the brand name on the heater. We have had our Kenmore electric hot water heater in the garage apartment for a few years now, and it has been totally-reliable so far.
Here's a 50-gal, 6-year electric:
http://www.sears.com/kenmore-50-gal...p-04232666000P?prdNo=3&blockNo=3&blockType=G3
They also sell a 55-gal, 6-year electric at the same price, as well as a 9-year and a 12-year in 55-gal.
In Philadelphia proper, according to the web site, installation by a Sears-retained licensed plumber is $270, drain pan is $90, $130 for expansion tank, plus $10 for haul-away. So about $800 for the whole job, including the water heater, installation, drain pan, expansion tank and haul-away. Plus tax. If you look at the web site, you can see the other options and get the precise installation cost for your actual city once you add the thing to your cart. If you call the number posted on the web site before noon, they offer same-day service.
Good luck, and let us know how it goes.
PS It looks like Sears will also hook you up with a $25 mail-in rebate if you are a PECO customer for installations before 5/31.