Decisions like this are tough, I just went through all of this 2 mos. ago, so I can share my experience and hopefully it will help.

I got the Nexstar trainer ($100) OS 46AX ($105) and Futaba 6EX FM ($230). The only thing I would have done different is get the Tower Trainer, not the Nexstar there's no need to spend $100 on a trainer, not that the Nexstar is a bad plane. My plan is to fly something I really like/want to fly for my second plane. I will take all the electronics and engine out of the trainer and put it into a (low wing, nice looking) second plane, so it was definitely worth spending the few extra dollars on the OS it will be a reliable, well performing powerhouse for my next planes. Personally, I get this funny feeling of confidence and pride when I see the OS stickers on the side of my plane, and to me that's worth a couple of bucks.

Another thought is the needle valve this is a part of the engine, connected to the carburetor, that controls the amount of fuel flowing into the engine. When you are tuning the engine, this is where the adjustments are made. The OS and the Magnum have a 'remote needle valve' meaning that the needle is mounted at the rear of the engine. This gives your hand/fingers about 3.5 in. of clearance from the spinning propeller, which can really cause some damage. The Thunder Tiger has an angled needle valve, which might be a little more efficient, but doesn't give you as much clearance from the prop (maybe 2.5 in). I just started my engine for the first time, and I am really glad I didn't have to put my fingers anywhere near the propeller. People might say there is the possibility of the back plate of the engine breaking during a crash because of the remote needle. OS has made a change so this doesn't happen. I don't know if Magnum did the same.

Both Magnum and Thunder Tiger require a glow plug, which adds ~$5.00 to the cost. OS comes with one. Magnum requires a smaller prop for break-in, which adds ~$3.00. This means OS is really only $5.00 more than Thunder Tiger.

The Magnum manual says that fuel with Castor oil, more expensive, is REQUIRED for break in (you can ask about this say it's not mandatory, but if the manual says to do it, you should. These engines are probably not made with the same tolerances as OS and maybe need the extra lubrication to get things settled so as not to loose performance in the long term, also maybe this is why people say break-in is a little longer). Thunder Tiger recommends this fuel. OS 'suggests' it, but also says you don't really need it in the manual.

Again, it's a tough call. You can go to the top of the page, under User Reviews > compare products > compare engines and compare all 3. You will find people really like all of the above options, but OS has been around the longest, and has the most satisfied customers.

OS is made in Japan, Thunder Tiger in Korea (I'm pretty sure), and Magnum in China.

Hope this helps!