It's an impossible question. Both are good, it depends where you're based, whether you're only flying on BA/LH (or oneworld / Star partners), what class you usually travel in, and what you like to use your miles for.

A few obvious points. BA Gold is oneworld Emerald which is better than Star Alliance Gold, as it gives you access to first-class check-in and lounges, whereas LH Senator only gives you business class lounges. BA Gold is actually closer to LH Hon Circle (level above Senator) which gives you access to First lounges on Lufthansa, but not partners. So purely from an alliance perspective, oneworld Emerald gives you more advantages across the rest of the network (ie when not flying BA), but the Star network is far larger and encompasses a lot more airlines.

Others can probably add more.