We're doing Universal + SeaWorld next week and here is how I ended up with my final choice...maybe my struggle can help you out.
First, we were getting a rental car for the week but I'd like to keep the $ down and would rather avoid the hassle at the counter with luggage. I like ME.
Second, we were thinking about Lincoln Town Car service but it was $109 just RT to Universal, RT to Seaworld would have put us at over $200 NOT including tips. Too much
Third, I thought about a shuttle but that's $16 pp for $128 (family of 4), too much.
Fourth, I thought about a public transport as it is the least expensive option but the time factor would drive me nuts.
FIfth, I entertained
Alamo at either WDW Car Care or the Dolphin and the prices were just bonkers.
Sixth, I thought about a MEErs taxi but they want $48 each way with tips extra, too much.
Finally, I discovered that the Grosvenor resort would pick us up at our resort the morning of our trip & drop us off after Seaworld on day 2. I feel the prices were very low considering the convenience and the current rates out there.

I just cancelled everyone else & booked 2 full days with a rental car at Dollar for $33.49/day with tax its $76.66 total. I will still need to pay $20 for parking but still come out lower than most other choices...besides, I can make a shopping trip since we're going to these parks my 2nd & 3rd days at WDW.
Good luck with whatever you choose.