TARDIS because I get to time travel in my fictional form. Today, you can meet my "historical romance" self--the one who inhabits South Carolina in World War One and nineteenth century Nebraska and hangs out with war heroes and prairie sheriffs.
I adore writing both. I try to write at least one historical a year--sometimes two--along with my contemporary series. The variety energizes me, shakes things up just enough to keep the creative juices flowing.
If you aspire to literary time travel like I do, here's three tips I've learned for flipping back and forth through the centuries.
1) Collect the verbs
Every era has splendid verbs that belong only to that time period. No one "coshes" anyone over the head in 2010, but no one "ditches" a date in 1845. Nouns are useful, but I find it's the verbs that really make a text sing.
2) Get the rhythm in your ears
While I find I can't read historicals while I write them (I'm too impressionable and then my voice starts to veer unnaturally toward whomever I am reading), I find it useful to watch movies set in that time period or listen to audiobooks. It helps me get the sound of the language firmly fixed in my head. The same is true for a contemporary--what movie sounds like my character? Which audiobooks have the same sense of place I'm shooting for in my current work?
This is a bit harder for historicals, but I often wade through on-line music services to find a soundtrack for my books. It's instant atmosphere, and can take me to another place in my head no matter where my body happens to be.
Sure, I'd love a TARDIS--or any other time travel device that would allow me to dress up in a 19th century ball gown and still make it home for my daily latte. But until The Doctor shows up at my door, I'll have to make do with the technology I've got.