Short answer: I would suggest you resume reading at Chapter 42 of the Webcomic, which is just after the conclusion of the Boros arc, because while the next several chapters will be familiar from the manga the deviations start to mount up pretty quickly.
As YLombardi indicates, this of course heavily depends on what currently constitutes the "latest release". It is also important to note that the manga starts deviating pretty heavily from the webcomic, though currently not in ways that fundamentally alters canon (though it's getting close). Most of the changes involve certain fights being extended and given extra details, as well as giving the non-essential heroes some extra exposition and screen time in other fights. The most recent manga release (as of my typing this) is concerned with a tournament that never occurs in the webcomic, though this tournament is just part of introducing the two major atagonist powers for the next arc; both such powers and said arc are part of the webcomic.
I will try now to point out when certain major arcs begin/end for the webcomic, up to the current status of the manga (said tournament storyline is closing up). Anyone completely unfamiliar with the series will, of course, find lots of spoilers in the subsequent.
- First chapters, introducing Saitama and his fights against Vaccine Man and the brothers, his "origin story" fight against the crab man, his dream, etc. are essentially identical.
- Saitama vs. Ordinary Mosquito, and introduction of Genos: Webcomic Chapters 5-6. Essentially identical between versions.
- House of Evolution: Webcomic Chapters 7-11. Essentially identical between versions.
- Paradise baldies and introduction of Sonic and Licenseless Rider (Mumen Rider): Webcomic Chapters 12-15. Essentially identical between versions.
- Joining the Heroes Association, introduction of Sneck: Webcomic Chapters 15-16. Essentially identical, but the manga/anime shows a few more tests Saitama does and gives us a better idea of where the tests are held.
- Saitama vs. Genos: Webcomic Chapter 17. Essentially identical between versions. This is one of the first cases where Murata, the manga artist, has a lot of fun with just plain drawing. An entire chapter of the manga is basically spent with Murata creating an intimately drawn flip-book for this fight. The anime basically could have just scanned in the pages for the Murata version of this chapter and called it a scene. The details of the fight are the same, other than that the landscape is not shown as obliterated by Saitama's partial punch in the webcomic (nor is any of the damage that Genos did shown).
- Genos moves in, second encounter with Sonic, Tank Top Tiger introduced: Webcomic Chapters 18-19.
- Saitama vs. Meteor; Silver Fang, Metal Knight, Tank Top Black Hole introduced: Webcomic Chapters 20-23. Essentially identical between versions, with a few minor differences, mostly concerned with Genos having an upgrade available. Murata has fun in the manga again, basically animating (in color) Metal Knight's arrival. The anime again could have pretty much just scanned in that chapter and called it a scene.
- Sea King arc: Webcomic Chapters 24-31. Essentially identical between versions. Same heroes are introduced, including Puri-Puri Prisoner and Handsomely Masked Sweet Mask (Amai Mask), etc.
- Boros arc: Webcomic Chapters 32-41. Nominal differences between versions. Saitama encounters Boros slightly earlier in the Webcomic, meeting him at the same time as Geryuganshoop. Geryuganshoop does not display a gravity power in the webcomic, and only ever use telekinesis to throw rocks and rubble. The final fight between Boros and Saitama is much longer in the manga/anime than in the webcomic. Saitama is not knocked to the moon in the webcomic at all.
That's as far as the anime gets, but the manga is at the start of the Garou arc. From this point on the manga starts to diverge fairly noticeably. For the most part this does not change anything in the canon, but as previously mentioned the manga gets this arc started with a martial arts tournament which does not exist in the webcomic. As such there are a lot of new characters and enemies in the manga, but so far none of them look to be important beyond setting things up (but we have to wait and see).
Spoilers beyond this point for those who have watched the anime but have not read the manga (or webcomic).
- King vs. Saitama: Webcomic Chapters 42-44
- Garou Appears: Webcomic Chapter 45
- Saitama meets the Fubuki Group: Webcomic Chapter 47
- Garou's hero hunt begins: Webcomic Chapter 51. The noticeable differences arise around this time, as who Garou fights and when is different. The level of interaction between King and Saitama is also substantially altered, because Saitama is at the aforementioned tournament in the manga, while King is not.
- Monster Association appears: Webcomic Chapter 55. Substantial differences between the manga and webcomic are present at this point, though the essential thrust seems so far to be the same.
This is essentially where the manga is now (which is at chapter 109, by certain reckonings). I would suggest you resume reading at Chapter 42 of the Webcomic, because while the next several chapters will be familiar from the manga the deviations start to mount up pretty quickly.
But these 55 chapters are only barely more than half of the webcomic's current length, as the webcomic is at chapter 109 (by coincidence) as of the time of this post. The Garou/Monster Association arc lasts all the way up through Chapter 94 of the webcomic, making it longer than the entirety of the story up to Boros. Chapters 95 and on have so far not introduced a clear arc, but a lot of world building, character development, and character backgrounds have been provided.