This response is probably too late for your purposes; but having just seen it, I thought I would respond.
In short, there are only two reasonable choices for the first dialect a beginner should study: Sahidic or Bohairic. If you are interested in Coptic from an academic point of view, intend to pursue Coptic extensively, or want physical books, I would recommend Sahidic. If you are interested in the modern Coptic church or community, are interested in the modern revitalization movement, or want just a light taste, study Bohairic.
Sahidic is the dialect normally recommended for language learners outside the Coptic Church for three reasons:
It was the earliest known prestige dialect of the Coptic Church, being the standard up through the beginning of the Islamic era.
It has few peculiarities not shared with at least some other dialect.
It is the only dialect with a large body of literature originally written in Coptic, as apposed to translations from Christian Greek literature.
There are a number of excellent academically oriented grammars of Sahidic, including the ones written by Lambdin and Layton. There is also a newer grammar of all six dialects written by James Allen, whose writing on Egyptian I have also found to be excellent, but very dense and not necessarily very approachable for those not very versed in linguistic concepts.
If you are specifically interested in the modern Coptic church and community, want to rely substantially on internet resources, or just want a basic exposure to Coptic, there are better resources for Bohairic, mostly created by Copts and targeted mostly at Copts wanting to learn the language, including children. I have searched less online, but recall some good reference sites and several sites offering introductory lessons with minimal linguistic terminology.
A substantial amount of Coptic literature consists of Biblical translations or Christian writings. I think the surviving Sahidic versions are older, but the Bohairic versions might be more closely aligned to the literal Greek text. These were separate translations and so have different text beyond the difference in dialects. To explore either dialect, expect to read a lot of the Bible in translation. I think there are also portions of the Bible in other dialects with differing completeness.
I would agree that a learner should focus on one dialect first, before branching out into others. These differences are fairly minor and often follow somewhat regular correspondences but occur in a large percentage of words. From what I recall, there are also some minor differences in morphology and syntax. Coptic has a lot of verb forms that are unlike even Semitic languages and need much study to understand. The differences between the dialects is partly due to different spelling traditions and partly due to what we assume were differences in phonology. Mastering the dialect differences would be daunting for a beginner already faced with a complex verbal system.
The differences between the dialects are, in my opinion, less then the differences between Arabic dialects but are perhaps roughly comparable to differences between the various Romance languages/dialects spoken within mainland Italy or within the Iberian peninsula. Thorough knowledge of one pretty much gives you an ability to read other versions, even if you may not understand all the subtleties.
If you have no ties to the Coptic community and have no preconceptions about what you want to study, I would base your decision on your learning style. The Sahidic grammars tend to be thorough and rely heavily on a linguistic approach. The Bohairic materials tend to be less linguistically thorough, but more user-friendly and practically oriented.