I currently own a 2004 Toyota with the 4.7L V8. Before this, I owned a 2007 Tundra with the 4.7L, and then a 2010 Tundra with the 5.7L V8. That 5.7 was a great engine. 381hp/401tq, with a solid 6-spd trans. For a "1/2" ton gasser, you can't beat it. It's rated to tow around 10,500lbs (depends on configuration). The big problem with the Tundra, and any other 1/2 ton, is Payload. Some people have claimed that since Toyota only builds one full-sized truck, they built the Tundra with some parts in line with a 3/4 ton. The axles, brakes, rear-end (10.5"), etc... However, when it comes to payload, you're still limited to around 1500-1600lbs (again depending on config). Where as the 3/4+ ton trucks have 2500lbs+ of payload capacity.
I'm in the same boat as you. I sold my '10 Tundra so I could afford my 1st tractor (on my 2nd now) and bought the Sequoia used. It's been a great truck, but it's limited to 6500lbs towing cap. Now that I have my new tractor, this truck is over-worked... tractor weights 5500 all said, trailer is 1500+.
I've been looking around at 3/4-1ton Diesels too. The best deals right now are on the 03-07 F250's with the 6.0L diesel. I spend some time on the Ford forums regarding that engine. As long as you spend a little bit of money, it's a great engine on a good truck. But, it can be problematic and costly if you blow the head gaskets. I looked at the GM's, but it appears that if you get the 04 & older Duramax, you have to worry about the injectors which are even more costly. But the 05 & newer are more expensive to purchase. I've never been a fan of Dodge products personally, but I hear the 5.9L cummins was a fantastic engine.
The problem for me, and probably you, is... Diesels don't like to sit. They like to be used. Especially turbo diesels. This truck (for me anyhow) will be used only when needed, as we have 2 other "commuter" cars to use. That started pushing me to an F250 with the 6.8L v10, because it has good power and is pretty reliable (spark plug & exhaust manifold bolts notwithstanding). The trade off is crappy gas mileage. I personally owned an 04 F150 with the 5.4L v8 and it was a dog. I have read that the 5.4L in a SuperDuty is every worse (because of the extra weight) and I can believe it. That's too bad, because the absolutely BEST deals right now on used, are F250's with the 5.4L. Even if you swap the stock 3.73 gears up to 4.10 or 4.30, it's still a dog.
So I started looking at a Tundra again, and for around $18k you can get an 07-08 with less than 100k miles. Personally, I'm not worried about miles on a Tundra, my current rig has 135k and all our Toyota's have been fantastic. But just be advised, the 5.7L has an issue with the Water Pump, so if you do shop for one, check for leaks or seepage. It's only a $90 part, and if you can do the work youself, you'll save $400 in labor cost. The 07+ Tundra's also have a problem with the AIP system (Air Induction Pump) which is a startup emissions control device they had to add. Basically it injects extra air during startup to burn off harmful emissions. However, some have been prone to collecting water and throwing a code, which causes the ECU to put the truck into "limp mode" and you have to get them replaced. There is a work-around and you can bypass the AIP's completely for just a few bucks. But, with all this said...you still have that payload issue. The only way around it (not legally tho) is to beef up the rear suspension with Air-Bags or springs, and put some better (E-Rated 10ply) tires. All said, you're gonna spend at least $1k to do this. The engine and trans can handle the loads. The payload issue may never be a problem for you though, as you might never exceed it. I've been a member on one of the Tundra forums for several years, and there are lots of stories of Ford/GM/Dodge converts who claim their Tundra is a better puller than whatever 3/4 ton they were leaving behind, with much better reliability. Could be a bunch of BS, but who knows anymore with the internet.
So, I'm still up in the air myself too. Rumor has it that the 2014 Tundra might come with an "HD" package that will address the Payload concerns, but that might just be a rumor. I think that all the new trucks on the market are a quality and capable product right now tho, so if you are buying new, it's a tough choice. I might just wait a year and see what all happens in the truck market