Edit: Correction on the tip choices, goofed up looking at the T12 instead of the T15
Tip availability and cost is about the same for both, T15 for the 951 and 1124 for the Pace TD-100. Pace may have a bit more buying choices and may be slightly cheaper depending on tip geometry, but YMMV. Other considerations:
1. The 951 is lower power (70 watts vs 90 of the ST-30)
2. The 951 is generally of worse build quality. The station itself is plastic, stand is thin gauge metal, etc. The Pace is metal and very thick gauge metal for the stand.
3. I'm not a fan of the handpiece of the 951. Firstly, tip replacement is a two step process - remove the grip first, then the tip from the grip. One step on the TD-100, just remove the tip. Secondly, the grip itself is foam (meh) on the Hakko and thick, with a larger grip to tip distance than on the Pace. The TD-100 accomplishes a much slimmer grip without the need for the extra foam insulation due to the air gap around the tip accomplishing this.
4. Temperature stability on paper is worse on the 951 - ±9f vs ±2f on the Pace.
5. Heatup times and therefore thermal recovery are better on the Pace as per most reviews I've seen of the 951 and Hakkos own graphs. Note that I don't have a 951 myself to test this on, but the easiest test that I've found that your trainer can do is to take the same style of tip on both (nice and clean of course), cut two same length pieces of solder, tie them in a hanging loop at the edge of each tip with stations set to the same temp (350c, say) and to turn on both stations at the same time. The one that drops first wins - there are other factors of course like overshoot and firmware bootup times, but eh, that's the easiest without more expensive tools :)
6. The main advantages of the 951 are the auto sleep that the ST-30 doesn't have (optional), and the digital readout, as well as some things like lockout that are of not much interest to a hobbyist. The digital readout is nice, but comes at the cost of taking longer to change temperature - easier to rotate a knob than to push or hold buttons.
Hope that helps!