IAMPETH's conventions started 67 years ago and were conventions for members, most of whom were in the business. Fifteen years ago, when I first attended, it was still a convention full of working scribes and there were few -if any- beginners. We were all there to participate in sharing of information on topics that were beyond the basics. There were 70 people in one hotel banquet room so it was pretty easy to meet everyone. The presenters were members who donated their time. Nobody presented more than one day or a half day. Members were also donating their time to publish the IAMPETH newsletter and build the website. As the internet made the organization more visible, the numbers at the convention grew and beginners started showing up at conventions. Beginners were always welcome. As the attendance has grown (250?) IAMPETH has begun offering more classes for beginners but the class size is quite large, so individual attention is limited. The presenters are top notch calligraphers, but you will not be spending a week with one person as you do at an international conference. At IAMPETH you get more of a smorgasbord rather than in depth study. The last couple of times I attended, I would sit next to beginners and offer tips. You can learn a lot from the old-timers if you cross paths with them. Students sign up for half day or full day classes and receive a large notebook full of handouts from all the classes.
I noticed on the website, IAMPETH is now referring to the convention as both a convention and a conference. It is held at a hotel and after you pay for your room and meals, the cost is comparable to the international conference which is held on a college campus where you stay in a dorm and eat cafeteria food. I believe that once registration begins, IAMPETH sells out very quickly. The international conference has more flexibility in numbers so early registration is not an issue. The only advantage of early registration is if you are intent on getting into a certain class that might fill quickly.
The international conference is in its 37th year. It was started by the Minneapolis-St. Paul guild and each year a different guild sponsors it. Each guild gives the conference a different name, so it is a little confusing but you can always find it by Googling international calligraphy conference and the year. You will not find LetterWorks in 2018. The conference is being hosted by the Seattle guild and I do not see a name on their website - yet.
Before the recent growth spurt of IAMPETH, the international conference was always much larger (300-400?). The classes are 12-16 people and the instructors are paid. Students have the choice of a one week class or two half-week classes. The in-depth study with a rock star is an advantage if you are looking for in depth study.
IAMPETH has historically been more focused on the pointed pen styles with a smattering of broad edge styles. The international conference offers a lot more variety in options. If you know you are strictly a pointed pen scribe, you might be much better off at IAMPETH. Broad edge scribes probably won't find what they are looking for at IAMPETH.
If you want to learn both pointed pen and broad edge, then you really need to attend one of each to determine which one is for you. The camaraderie is identical at both. People like Heather Held and Michael Sull have been frequent presenters/instructors at both IAMPETH and the international conference. In my 15+ years of attending both - I have met plenty of other people who have attended both and would not rate one better than the other.
Often times people make the choice by the cost of the plane ticket. I have never heard of anyone being disappointed in an international conference. Likewise, I have never heard of anyone being disappointed in IAMPETH.