If you had to pick one which would it be and why??

What time of year are you planning to travel?

Summer - probably June.

Then either!

If you like watersports (windsurfing, sailing, kitesurfing and waterskiing) then Lake Garda is a good choice, specifically Torbole, but other towns have options too. It's usually very busy around all sides of the lake, but June will still let you find some space. I enjoy Riva del Garda (well-visited but still a good atmosphere, great restaurants and bars, northern end of lake so good for day trips into Austria, Germany and even Venice) Limone sul Garda (dramatic scenery) and the smaller Torri del Benaco (small village atmosphere, pretty harbour aream, castle). Sirmione, on the souther point of the lake, it also very pretty, but can get jammed up with day trippers between Verona and Venice.

Lake Como, even though there are also watersports there (including scuba diving), has a far more relaxed feel I find. Plus the romance factor is a little higher on Lake Como (IMO) with Bellagio, Varenna, the gardens of Villa Carlotta, if that's what you're looking for.

Probably didn't help you choose, but there are points about each depending on why your travelling there.

Fodor's is a highly self-selective forum, and the demographic of its frequent posters (age/income, etc) isn't all that wide, so the votes of Fodor's posters as to what they would choose and why can be misleading.

If you said more about why you want to go to an Italian lake while you are in Italy, maybe people could steer you toward the spot on the lake that stands the biggest chance of meeting your desires. Lago di Garda is huge and varied, and Lago di Como has quite a range of accommodations when it comes to price, so you'll get better advice if offer more about what you like rather than ask what people here like.

Now that is funny!
So, completely disregard any of the opinions you may get here since the demographic may not match yours. After that, I hope you get some really good input. I was going to comment but I am afraid I may represent a demographic that is not valid.

Bob: Z did not say your input was not valid.

But it IS true, I think, that one should know a bit more about the age, tastes, and travel style of those who offer advice.

Agree that it is impossible to give an opinion of offer meaningful advice if the OP does not reveal more details.

True, but a poster might be asking what YOU think. You don't need to know that I'm 40 or have decent money to spend or don't like waterskiing to tell me which lake you prefer and why. That's why I said why. Is one lake prettier, or more touristy, or have more restaurants, or easier to get to, or have more places to travel from, or have better accommodations. I don't know exactly what I prefer, that's why I'm asking the difference between the two.

Regardless of what I may like, if you could only visit one of those lakes before you die, which would YOU choose and WHY? Once I see the pros and cons then I can make my selection based on the data and what I'm in the mood for. Maybe I never would have thought to ask about something until I've heard about it. That's why us infrequent travelers rely on you frequent travelers to help us out. I know not to ask someone to create an itinerary for me with giving no data, but I've narrowed it down to two beautiful lakes relatively close to each other the same time of year and am wondering what you all have seen to make me choose one over another.

That and I love to daydream about these kinds of getaways that I'll go on someday and live vicariously through you all!