Geist is well worth buying, snap it up if you see it in a sale. It's perfect for creating drum kits from samples. There are 8 "engines" each of which can host a kit consisting of 16 pads, and each of those pads can host multiple layers of samples. It has excellent onboard effects (filters, EQ's, delays, modulators etc) which you can apply to individual layers, pads or the whole engine itself. You can also do all of your pattern sequencing in there, and each track of each pattern can be automated any way you like. You can also arrange your whole song in there, as long as it doesn't contain any time sig changes. Otherwise you can just trigger the patterns via MIDI (or trigger individual pads if you want to do all the sequencing in Sonar).
Battery 4 is also excellent - it's more powerful in terms of creating drum kits from scratch, and the per-drum effects are top notch. It's only drawback is that it has no pattern sequencer. I'm glad I have both Battery and Geist and I use them both, but if I had to choose between one and the other I would probably choose Geist.