Stickpacks are simpler for the beginners and less experienced because there is no danger of shorting.
Saddle and side x side packs have a lot of advantages.
1. Lowering the overall CG. battery slots in most chassis will allow the saddle pack to sit lower, and will lower the CG. Lower CG means better handling.
2. saddle packs allow for cell equalizing using tray dischargers..something most of us use.
3. Saddle packs allow for indivual cell testing and removal. Sometimes 1 single cell in the whole pack dies, but you won't know which one in a stickpack. In a saddle pack you can test each cell with a voltmeter and you can remove the dead cell easily.
4. Generally the connectors used in stickpacks have higher resistance and will melt under high loading.
Tamiya does not make batteries, but they do have (SANYO)RC3300HV cells with tamiya stickers on them.