What is the most efficient way to make troops with gem pack or with the bootcamp pack?
If you've unlocked T4 then use the ratio 1 T4: 8 T3 to speed up the training of troops. Personally I'd take the boot camp pack and invest in speeding up the troop type that you are
Range, inf, cav or siege. It's best not to invest in siege, their "durability" is half a normal troop. The gem pack can also be useful~ it's a hard choice
You must realise Lords is all about efficiency and micro-management. To be able to answer your question, you must look at a few factors pertaining to your situation.
What tier are you training? Each tier takes x2 more time to train per-troop than the last.
What's your Barracks capacity? You MUST train at least 24 hrs worth of troops if you want to speed using gems! Anything lower is woefully inefficient, and, frankly, a waste of gems.
This is because speeding costs are non-linear when dealing in gems, tho they begin to plateau at 24 hrs (300 at 3 hrs vs 650 at 8 hrs vs 1,000 at 15 hrs vs 1,500 at 24 hrs vs 4,400 at 72 hrs)
When training with speeds, Barracks capacity is irrelevant, as long as you have enough as not to waste any speedups. This is because training times increase linearly when using speedups (2,000 T4 take twice as long as 1,000, and half as long as 4,000).
Next, look at how much you're willing to spend. Both packs are released almost-weekly; Gem Packed is available in 5/20 iterations, and Boot Camp in 20/50 iterations.
Gem Packed gives you more bang for your buck if you train efficiently, as explained above. Boot Camp is 20 days for $20 and 40 for $50 (excluding gems given), but its rss and chests are something to consider if you're low. Its Training Speed Boost (20%) is also a big help if you don't already have a few.
All in all, I'd say to be optimally efficient - buy Gem Packed (tho I'd recommend both if you have the $). You can also use the gems to help with your research and get your Watcher to gold.