Even tho I'm still relatively new to the hobby, I remember hearing that there's a lot of L.E. activity in the Mission Valley area. However, the same could be said for the Downtown area, too.

Mv

one you may not have considered:  UTC La Jolla.  I'm an OC guy and I know several others that will routinely go as far as UTC and sometimes Mission Valley but she would have to be an extraordinary once-in-a-lifetime experience to get us to go all the way downtown to Gaslamp.  Local SD guys may see it differently because less driving for them.

Being a local SD hobbyist, I would opt for MV or UTC since parking sucks in the Gaslamp or you pay to park.

Lots of free close parking

Some people who comment are not going to see you, so it does not matter.  No offense boys.  I have clients to will pay for my room where they want me to have an incall & supply a deposit to book.  Try prebooking in advance & the guys who book ask them where they prefer you get an incall.  I prefer to accommodate the clients who are actually booking.  

MV is a LE hotspot and the hotels are very aware of things.
Many now have parking pass requirements where the license plate and make/model is required which is added cost, time and lack of discretion if you are looking to have more than two or three dates over two days.

Downtown is more upscale. MV is touristy and low rate motels. Also, more to do downtown.

Mission Valley the best. EZ access off the I-8 and with plenty of EZ parking which hobbyists luv. Some very lovely new hotels with reasonable rates, not going to get anything reasonable DT. Do not listen to the fake news that MV a LE hot spot!! Based on what?? Just because 2 cops decide to have coffee there does not mean there's a sting. LMAO. LE has more presence DT than anywhere. I hate DT, no street parking, and very expensive parking. Many hobbyists I know hate DT. Cheers.

A few agencies have tried it in the past, because the agencies do better near Freeways for easy access.  Guys that book with the agencies are generally looking for a quick in and out (no pun intended), not to tie up extra travel time.  The higher volume the hobbyist,  the more efficiency he wants, IMHO.  Even MV is losing some agencies in the last year or so.  UTC is the preferred location.   From OC, its often a shorter drive, time wise, to go to UTC than LA because of the difference in the traffic patterns.

My two cents here.....
As someone in the hotel industry, MV is not where you want to be. While it may be more budget-conscious, there are more potential problems there. My security supervisor routinely checks thru BP and CL and can notice any photos taken from the rooms (curtains, carpet, bedsheets, etc) and sets up dates with the ladies and has them busted. There are plenty of places that are just as convenient as MV from Mission bay up to La Jolla for all sorts of fun to happen.  
Happy hunting!!

Downtown at night can be a total pain in the ass when it comes to parking.