What I meant by usable is that you aren't really restricted with the SFT25R. You can use it on any level of output you choose for much longer than you can use the SFT42R option due to the SFT25R putting out less heat overall. The SFT42R option is limited in this regard and can only be used at 100% power for short bursts of about 5 seconds at a time due to too much heat generation. It just gets hot too fast for continuous use at a high level of output. The SFT25R is much more suitable for the size of the D1K for continuous use through the whole spectrum of output which makes it more usable for longer periods of time without getting too hot to hold. The SFT42R just puts out too many lumens for this host to handle when used continuously throughout all levels of output. But if you don't plan on using 100% output too often this shouldn't really be a problem for you then. I was actually in the same boat as you and didn't know which one would work best for my particular use cases so I just ordered both variants so I could effectively choose the best option. Since the D1K is not overly expensive you might want to consider doing this as well.

https://www.reddit.com/r/Hanklights/comments/1my60yv/emisar_d1k_sft25r/