In my experience, gel tabs tend to be a bit more reliably potent than paper, but that's certainly no guarantee. As somnilicious says, there's not really a good way to tell without trying them, or talking to someone who has tried them.
It entirely depends on how strong they're made. LSD is active at such a tiny dose that you could fit about 50 standard doses on one piece of blotter paper, if you wanted. Likewise, someone could make a gel tab with barely any LSD on it.
Just as a note, a typical blotter can hold 3+mg if done correctly. Gel tabs can hold more but this doesn’t mean gel tabs typically have more.
From what I’ve seen it appears many rave about gel tabs but with anything popular it’s very easy to make some copycats. I personally stick to WoW blotter, only occasionally eating gels, liquid or printed blotter if someone I trust really talks it up.