Just got back from a Labor Day weekend trip and tried both on consecutive nights. Personally, I prefer Julian Serrano. This is not to say that both aren't good restaurants, though.
Ambiance/Decor: Julian Serrano had a much more upscale feel to me, while Jaleo was much more casual and whimsical. Jaleo was certainly much louder - it's open to the main floor and has lots of hard surfaces and the music can be loud. On the other hand, the people watching is pretty good - I was there around 7:30 p.m. and was watching the people come in from the pool, as well as, people bringing in pizza to eat in the lounge areas outside of the restaurant. Julian Serrano is better for a conversation with a dining companion - it's darker and more upscale, but not stuffy at all.
Service: Julian Serrano had better service - more polished and professional than Jaleo. The Julian Serrano server asked how I wanted my tapas paced and in what order and was able to deliver on my suggestions. The team at Julian Serrano was also very good - refills of water, clearing of plates, etc. Jaleo's food came out randomly based on whatever was ready at the time, I guess. Jaleo was also slower with longer intervals between refills of water, serving of tapas, delivery of the bill, etc. Again, not to say that the service at Jaleo was bad - it was fine - just not as good as Julian Serrano.
Food: I enjoyed the food at Julian Serrano better, but I think that's personal taste. My only experience with tapas is the 2 weeks I spent in Spain (Madrid, Barcelona, Granada, Sevilla, etc.) eating in tapas bars. Jaleo's food seemed a bit more authentic to me - the flavors were more in keeping with what I remembered. However, I enjoyed Julian Serrano's food more because I found the flavors were bigger. Spanish food in Spain seemed a bit bland to me. The Huevos Estrellados at Julian Serrano really hit the spot with me, as did the chorizo with mashed potato. I can't really think of a standout dish from Jaleo, except for the desert. The flan with oranges and cream was fantastic. It certainly beat the almond cake I had at Julian Serrano. I had high hopes for the bacon wrapped dates at Jaleo, but I found the sweetness of the dates to overwhelm the bacon, which is hard to do. The croquetas Jamon Serrano were o.k., as was the carne asada, but nothing to write home about, really. I had pan con tomate with manchego cheese at both places, and I found the bread at Jaleo to be soggier than at Julian Serrano. One other thing - Julian Serrano has non-alcoholic sangria, which is really just fizzy punch, but it's nice to be able to enjoy something in keeping with the meal for people who don't drink.
Value: Julian Serrano was a better value for me. I was completely stuffed and well satisfied and the bill was around $60 for a single diner. Jaleo was an $80 tab, and I wasn't nearly stuffed. Portions at Jaleo were much smaller, and each dish was rather pricey.
3708 Las Vegas Blvd S, Las Vegas, NV 89109
I haven't been to Jaleo. But we got to eat at e (inside Jaleo). And simply based on that (plus our experience at China Poblano) I'd pick Jaleo. Andres is excellent.
I have eaten at Serrano though and it's no slouch. We had the fries with fried egg and chorizo (among other things) and it was to die for. Classic "stoner" food. ;)
But from what we saw people eating at in Jaleo. I'd pick them.
Also, when we were eating at e the chefs told us that Joel Robuchon was sat outside at Jaleo. So who am I to argue with Joel Robuchon! :)
I had been in both restaurants and I can tell you Julian Serrano no dout is the best. Not to talk about the restaurant itself, but the food presentation and quality is cpmplete wonderful.
By the way Robuchon had been many times at Julian Serrano and he loves the fresh Calamari.
Better than everyone is if you can have your preference base in your own taste.