That's not the only difference, 4G usually has significantly lower latency than 3G. This difference can (very) noticeably affect website loading times, for example, and also on-the-go multiplayer games. I don't have exact numbers right now, sorry...
Indeed, the round-trip-time (RTT) is much improved on LTE. For such a device, it makes a lot of sense to use LTE.
Usages that benefit a lot from an improved RTT:
SSH, multiplayer gaming, web browsing, VOIP, video game streaming
Usages where RTT performance doesn't matter too much:
Video streaming, audio streaming, large file downloads
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Does the EU 4G one support US voice only networks? If so I'm fine with that. I was hoping to be able to get rid of my phone and carry less stuff.
I had a look at the specs and frequencies here:
http://www.gemalto.com/brochures-site/download-site/Documents/M2M_PLS8_datasheet.pdf
The important part is:
PLS8-E:
Penta Band LTE: 800/900/1800/2100/2600 MHz;
FDD-Band (20,8,3,7,1); Tri Band UMTS (WCDMA):
900/1800/2100 MHz; FDD-Band (8,3,1);
Dual Band GSM/GPRS/EDGE: 900/1800 MHz
PLS8-US:
Quad Band LTE: 700/850/AWS (1700/2100)/1900 MHz;
FDD-Band (17,5,4,2); Tri Band UMTS (WCDMA): 850/AWS
(1700/2100)/1900 MHz; FDD-Band (5,4,2)
Quad Band GSM/GPRS/EDGE: 850/900/1800/1900 MHz
You will have to do some research on the frequencies used by each mobile phone operator in the US.
I believe that at least GSM 900 must be supported by at least one of your operators. It would mean you will be able to place phone calls and send/receive SMS.
edit: I had a look here:
https://en.wikipedia.org/wiki/Cellular_frequencies
It looks like you really don't have any GSM900 operator in the US
However, it looks like you have Sprint on LTE800. It seems to be their only low frequency for LTE, which could be good news since it might mean they are relying heavily on it for 4G coverage. However, I think the Pyra module doesn't do VoLTE (4G voice over IP), meaning for phone calls we're likely to do a CSFB (circuit switch fall back). This requires 2G or 3G access to establish a voice call. This won't be possible with Sprint's GSM and WCDMA frequencies.
And having traveled quite a bit, I can tell you the PLS8-E will be usable in almost all of Europe, Africa, Asia. It really is the "most-worldwide" option for LTE.