Here's my take having owned some of each.
Chevy used a TBI engine from 87-95. They are really wussy. The TBI flows about 390 cfm, the intake ports have big swirl vanes cast in them, and the cams are tiny... like 167 degrees tiny. They also have faulty valve seals that leak. Its not a reliability issue, they just have a nice puff of blue smoke to let you know that the engine started :) 96-99 trucks have the Vortec heads. Yes, they tend to leak at the driver's side rear water passage gasket. Its not a big deal, just fix it with FelPro gaskets when they start leaking. The engines are great. The Vortec is the single best-flowing small block head chevy every made. Good stuff with a ton of potential. Late 99-present trucks use the LS1-based 4.8, 5.3, and 6.0. They are all fine engines, but stick to the larger displacements if you tow a lot. The 6.0 makes nice torque, but it likes to drink gas fast. I forget what I actually got from mine, but it was 4-5 mpg worse than the EPA estimates.
Fords use the 4.6 and 5.4. Both are not beacons of power or torque, but they have proven to have VERY long, trouble-free lives. There aren't any specific updates they made over the years, but 99 they switched to a different intake and head configuration. Some repairs can be a little tough to reach, but not terrible. I just bought a 98 F150 with the 4.6 and it had 123k on it. I didn't hesitate for a second. I've had a few that went 250k+ without so much as a sensor failure. Pretty reliable pieces. They're wimpy, but I didn't hesitate to use a 99 4.6L F150 to tow 6000 cross country from L.A. to Rochester NY and back twice with my foot to the floor. I worked it hard (had to over the rockies :) and that truck is still going strong with 200k
Just an idea... pre-99 Chevy 1/2 tons could be ordered with the 6.5 diesel. They should be updated with a remote injector computer mount and a couple other upgrades for reliability, but they make mounds of towing torque and get up to 20 mpg. They're also just a chip, intake, and exhaust away from pushing 300 hp/500 tq. Since diesel is about the same price as gas these days, 20 mpg sounds pretty nice. You could also snag an early Duramax for pretty cheap.
Opinions:
GM gets the win for rear seat comfy. Ford gets the win for longevity of engine. GM seems to fall a little short in the electronics reliability department. I have an 04 Duramax that keeps going to CD3 every time I turn the key on, and the dash information center keeps flickering and showing random characters. I had an 98 6.5L that kept blowing rear brake light bulbs. GM gets the win for a more comfy ride, but only by a little. Ford gets the win for seats that look uncomfortable but are nice to ride in. GM gets the can for seats that look great but make your butt fall asleep on long trips. I'll take the vinyl bench in my 98 F150 over the leather buckets in my dad's 09 Duramax. Seriously.
Other little notes... Ford's F150 was old body style up to 96 with no third door available. 97-98 had a third fold out door on the passenger side. 99-03 had four doors with a rear door on both sides.
I also take issue with the cheaper to repair down the road thing. I used to maintain a fleet of up to 200 trucks from all three, gas and diesel, 1/2 ton up to 1-ton. Start to finish, a diesel truck put WAY more money back in our pockets. Reliability, longevity, resale value, maintenance, etc. Its true that oil changes are more expensive and more frequent, cost of some parts is more expensive, and there is always the scary and expensive injector pump fear, but overall, it will cost less start to finish than a gas truck. Don't avoid one because they're more expensive to maintain/repair. If anything avoid one because they will be more expensive if they are problematic. If you're a good shadetree mechanic, don't hesitate to buy one.
I would also (if you're thinking about a GM) strongly urge you to go 3/4 ton if for no other reason than the 4L80E tranny. The 4L60E in the 1/2 tons is just not that great. Expect it to die around 120k if not sooner. The Ford trannys are only marginally better, but I haven't had any trouble from mine.