What are the options for installing ADS-B OUT in a Cessna Citation to meet the regulations?
It depends on what kind of Citation you are talking about.

I found a presentation by Textron(ppt) that discusses ADS-B installation for various models. Textron is focussing on creating Service Bulletins (SB) for installation of ADS-B on recent models (up to 15-20 years old).

For the Cessna Citation a SB is available for the following models:


Pro Line 21


CJ1+: SB525A-34-90
CJ2+: SB525A-34-41
CJ3: SB525B-34-26
CJ4: SB525C-34-09 R1
Encore+: SB560-34-163
XLS+: SB560XL-34-76

Sovereign


Primus EPIC: SB680-34-33 R1

Citation X


Primus 2000: SB750-34-58



For the following
Cessna Citation SB's are under development (when the presentation was given)


Ultra, Encore, XL, XLS


Primus 1000; expected Q2 2016; Prerequisite: Universal FMS with SBAS LPV

Mustang


G1000; expected Q2 2016



It is possible to install another transponder than advised by Cessna. However if it hasn't been done already, you have to go through a Supplemental Type Certificate (STC) process for that which makes it rather expensive. It also wouldn't be supported by Cessna, which will impact the resale value of the aircraft. You may offset the cost of the STC by offering it to other Citation operators, but then you are suddenly in a whole different kind of business. 

Other considerations to take into account are support of future operations, mainly LPV approaches using the same GPS receiver. 



Since all offers seem to prefer the Garmin GTX, I assume the aircraft is a Cessna Citation CJ1 or CJ2 with a non-Collins FMS. Cessna has a STC for this aircraft that is based on the Garmin GTX.