I just want to address a few of your questions real quick. I purchased a 500gb Sandisk SSD, and have been very happy with it. It won't increase your download speed, but it will increase your patching speed. I don't know what games you play, but for certain games, this can be very useful. For example, Ark will sometimes release a 250mb update that takes less than a minute to download. However, that update goes into the game file and messes with (for example) 9 gigabytes worth of game data. This takes forever with a legacy drive, and you will see significantly higher speeds with the SSD.
People give conflicting information as to whether or not gameplay is affected by an SSD, but I can tell you personally that I experienced a performance boost in a select few games. The main one I use as an example, is Just Cause 3. This is a high graphic open world game if you aren't familiar with the series, and it requires a huge amount of rendering. With my old legacy drive, I encountered smooth frame rates, but occasional stuttering. I don't get that with the SSD.
As for what particular SSD you should buy, I would first recommend a Samsung 850 evo. If you can't afford that, my next suggestion would be to just hunt for a deal on a reputable brand. If you've narrowed it down to the two drives you mentioned earlier, I would go with the Sandisk. Like I said, I have the Sandisk Ultra and I'm very happy with it.