As we all know, network performance under Windows 98 is not known for its speed, but I had an opportunity to test a few different network cards under Windows 98. This isn't a comprehensive analysis of every possible option and scenario, but it did yield some interesting results.

My test setup is as follows:
Windows 98 SE
Asus P3B-F Motherboard - PCI bus @40MHz
Pentium III 850Mhz - Slot 1 Overclocked to 1020MHz
512MB PC133 RAM - Running at 120MHz 3T

For Storage since the built in IDE controller is only ATA/33, I used the following to make sure the IDE interface wasn't bottlenecking the performance.
Promise FastTrak TX2000 ATA/133
Maxtor 7200 RPM IDE Hard Drive

Throughput tests were done via a CIFS share to a Linux VM running Samba. I copied a ~750MB ISO file to the desktop, timed it, and calculated the throughput.
I had 3 NICs available for testing:

3COM 3C905C-TX-M
Chip: 920-LU00 / 40-0577-004
Driver: 3Com 3C90x - 04.31.0000.0 - 04.31 / 2002-08-13
Speed: 100Mbps
Throughput 10.1MB/s (84Mbps/100Mbps)
~85% of a 100Mbit connection. Not great, but its 100Mbit NIC. Let's be honest, you're not using this because you expect it to be fast.

Intel Pro 1000/MT Desktop Adapter
Chip: 82541GI
Driver:: Intel Pro/1000 - Version 22.1 / 2017-03-25
Speed: 1000 Mbps
Throughput: 13.5MB/s (113Mbps/1000Mbps)
This is ~35% better performance than a 100Mbit NIC. If you already had a 100Mbit NIC, upgrading to an /MT wouldn't do you a lot of good. Still a good option for a Windows 98 machine nonetheless. Tweaking other settings in the Intel PROSET II utility didn't change the results.

Intel Pro 1000/GT Desktop Adapter
Chip: 82541PI
Driver:: Intel Pro/1000 - Version 22.1 / 2017-03-25
Speed:1000 Mbps
Throughput Default Settings: 15.2 MB/s (127Mbps/1000Mbps), 12% better than the Intel Pro 1000/MT
Throughput with Interrupt Moderation Rate=OFF: 20.7 MBps (173Mbps / 1000Mbps), 53% better than the Intel Pro 1000/MT
This is a nice boost 50%+ speed boost over the MT, though obviously nowhere near Gigabit speeds because of the limitations of Windows 98's network stack.

Final thoughts:
When looking at the different options for PCI NICs, there isn't a lot of information regarding the differences between them, especially when they use very similar chipsets released around the same time and use the same interface--ie an MT and a GT. The cards look nearly identical, with the only notable difference seems to be the slightly different chip on them. What does this mean for us? Given the choice, go with the GT.