A pack is best if you want a set of portraits for one or a few pets and you do not want a subscription. $9.99 gets you 40 portraits across four themed packs, and they never expire.
Unlimited is best if you will make many portraits in a short time, for example a party, a holiday card run or a week of posts. It is $6.99 a week and renews until you cancel in Google Play.
Heartgild Pro at $99.99 is for people who make portraits for others, like groomers and shelters, and need 600 portraits.
If you are not sure, use your free portrait first and then decide.