TransFlection is a technology from Panametrics,and it is still developed from transit-time principle. For detailed difference, it should be a business secret, so now there is no more details about it.
Since its Natural Gas inlet to the refinery I am sure refinery is paying for the gas being supplied i.e. custody transfer application. if that is the case I would recommend fix type device with best possible accuracy. Multi Path Insertion Type on a spool piece would be highly recommended. if accuracy is not a big concern then single path could also do. Portable has no meaning on this application provided it is just a monitory flow measurement that to be periodical checking.