CJ Premier 17LS vs Sonic Frontier Line 3

It's a hard call. You really need to hear them side by side in your system. IMO the CJ is a more subtle preamp allowing you to hear detail, instrument and singer placement and inner harmonics. In some systems the top end sounds ever so slightly softened (not to be confused with being rolled off or compressed). Otherwise, it is simply listening to music as it should be. I like both, so it comes down to your ears. Take the time and listen to them.

Though they both have tubes, they're different. Whats the rest of your system like and what are you hopeing to achieve?

As a CJ17LS owner, I can heartily recommend it. Soundwise, it's an elite performer and gives you much of the performance of the heralded CJ ART. However, there are a few caveats.

I believe CJ kept the basic design of the ART, but, in order to bring the price down, some tradeoffs were necessary. The primary tradeoff is that it is not designed to drive long cable runs or high capacitance cables. If this is a factor, try the CJ 16LS or the Sonic Frontiers you mentioned.

In that same realm, there are other excellent preamp makers with slightly different features/sound to consider - ie. First Sound, Joule, ARC, Pass, Placette and Aesthetix. Besides sound, I find that a preamp decision can also be heavily influenced by other features such as:

HT Bypass option
Tube/SS
Amount of gain
Remote
Type of volume control
Current production model or not
Balanced versus Single-ended inputs/outputs

Make sure you consider all these aspects before making the decision best for you.