Hello,
I am not quite sure if you have already found your way.
its very difficult to say which one is better without knowing your business scenario.
However, I have worked on both the scenarios and below are the few key things you have to consider.
Do you know which system will be used to generate the BOL/Packing List ?
if you are going for complete automated solution now or in the near term it will be good to use TM-GTS for triggering export declaration otherwise you can go with GTS.
if you are going to efile - then GTS will be able to send the ITN back to TM via standard service which will be further used in GTNexus/any system you use for your bookings.
Also, wanted to let you know, if you will have to do the SOAMANAGER configuraiton to activate the services between TM and GTS.
Please do let me know if you have any additional questions.
Regards
Rajesh