I need to connect RCA outputs to some M-Audio monitors that accept either XLR or TRS 1/4". I see cables at B&H for both configurations. Is one preferred? Thanks for any help.

Nah, get what you want. XLRs are considered 'more professional' and have a locking mechanism but a TRS (1/4"?) is electrically the same.

Buy the cheapest, best made cable. I'd buy a good looking 1/4" cable than a cheezy XLR.

I need to connect RCA outputs to some M-Audio monitors that accept either XLR or TRS 1/4". I see cables at B&H for both configurations. Is one preferred? Thanks for any help.
On the M-Audio speakers I see...
http://www.bhphotovideo.com/c/product/386231-REG/M_Audio_9900_40785_00_BX_5A_Studio_Reference.html

... speakers that have XLR and 1/4" inputs, the XLR is for a balanced input, and the 1/4" can be balanced TRS or unbalances TS.

Since your are coming from an unbalanced RCA connection, the correct connection to the speakers would be the unbalanced 1/4" TS plug.

You can use this adapter on a regular RCA cable:
http://www.bhphotovideo.com/c/product/469430-REG/Monster_Cable_600464_CableLinks_Adapter_RCA.html
(you don't need the expensive Monster version)
You can get these at a local electronics store or even Radio Shack. It should cost around $3-$7. They might even come in a package as a pair.

Or you could use a cable with two 1/4" plugs and this adapter:
http://www.bhphotovideo.com/c/product/408120-REG/Hosa_GPR104_1_4_TS_Phone_Female.html



(Note, it is possible to connect an unbalanced line to a balanced XLR connector if it is wired properly [they can be purchased], but in this case the RCA to the 1/4" TS plug is the best option.)

Thanks very much for the information Jack and Andy.