Both of these cases are improved versions on the 300 UltraMag case. The 300tomahawk gains you about a maximum 6 grains more powder. With heavy for caliber bullets (220 or 240g) it offers a little but not much more velocity. Olspark found that the increased velocity was nice but the accuracy started to dissipate. The sweet spot in accuracy was at a velocity obtainable with the standard case. The other consideration is the cost of custom dies, fireforming your brass, etc.
The 338tomahawk looks to be the optimum caliber to case capacity ratio for that big case. He's still burning over 100g of powder every time he pulls the trigger but is getting close to 3200 fps with the 225accubond. He's not at max. load yet. There is an advantage with this over the 338 Ultra mag because the 338 RUM case is shorter than the 300 RUM. The tomahawk is built off of the longer case.
If you're looking at shooting less than 1600 yards I don't know that the modified case will benefit you. (I'm not sure that it does over that yet!) If you looking at shooting at under 700yds, definitely stick with the standard case. I would suggest if you handload to try the 190 Sierra Match King in the 300. You can drive it at a good velocity, accuracy will be excellent, and the b.c. is plenty high. Just make sure you don't shoot them through the shoulders with it, unless you like your deer meat pre-ground
The best advice I can give in regards to long range practice is to shoot a lot. Find a range that has benches out to 500yds, farther is better. You need to know what your gun will shoot as far as you want to shoot it. That's why we shoot as many groundhogs in the summer as we can. It's a great learning experience in wind doping, mirage reading, and trigger control. The good thing is that here in western PA we don't have to deal with that in deer season. Our wind has averaged 3.7 mph in all of the readings I've taken for the past 2 years while long range hunting. The other thing you need is a GOOD rangefinder. Most 1000 or 1200 yd models will work to 500 or 600 yds, if you're going to shoot farther, get a Leica 1200 or there new geovid bino's. From experience the geovids are an amazing piece of equipment. But be forewarned that if it's the lease bit foggy, the best of "lazers" don't work. And if conditions aren't perfect, they won't range to 1000yds. After that you need an optical rangefinder such as the Wilde or ones from Barr & STroud. They work no matter what.