Option 3a 5G data will be sent by SCG (Slave Cell Group) Only, that is only through gNB, while in option 3x SCG Split bearer is used, that is part of the data can go through eNB in addition to gNB.
If you are using higher frequency band for 5G than that of 4G then the footprint of gNB will be smaller than that of eNB. And in case you end selecting option 3a then at gNB cell edge Ue might end up having lower throughput than that if its being served eNB at that point.
But if you are using option 3x then SCG split bearer functionality will keep the Ue served by eNB.
attached picture is taken from:
https://www.3gpp.org/ftp/Information/presentations/presentations_2018/2018_10_17_tokyo/presentations/2018_1017_3GPP%20Summit_03_TSG-RAN_Bertenyi_Nagata.pdf
