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Just to re-inforce the advice enroutesigle gave you, there is no road around the lake from Panajechel to San Pedro. Hope you arrive in clear weather for some great views of the lake as you zig-zag down the mountain from the highway. It'd be worth a stop if you come to a place to pull over and relish your new home on the way in. My very last visit to the lake was "several" decades backPana was a sleepy village whose restaurants offered little else than typical guatamaltecan dishes.
I hope you have a map of some sorts so you can see the towns we are talking about. Maybe you can find something online. Or find a Lonely Planet, which is full of maps. Or buy a highway map when you first cross into Guatemala. The security risk isn't worth it. So that means whether you cross at Cuauhtepec or Tapachula you will end up on Hwy 1 and need to turn off it down to the lake as enroutesiglno notes.
We have driven several times between Santiago Atitlan and San Pedro la Laguna over the last couple of years - coming in either direction you just need to stop in at the police station, tell them where you're going and they give you an escort for the few kilometers that are the only security risk along that stretch and they know exactly how bad it is and where the bandidos hang out, so they don't need to be asked twice.
Eyeballing it on the map, I'd say that the distances are pretty similar going the coastal or highland route from Tapachula. The advantage of going down the coast highway then up through Cocales and Santiago is that you miss out on all those heart-in-mouth hairpin bends coming down from Santa Clara. Plus the road from Cocales to San Lucas is one of the prettiest in the country, imho. Yeah, I surely am going to need one.