TL;DR
YOPmail and Guerrilla Mail are veteran disposable email services from the 2000s era. Each has operated for over fifteen years. YOPmail offers longer email...
Aspect
YOPmail
Guerrilla Mail
Inbox Privacy
Public inboxes mean anyone who enters your address can read your emails. There isn't a private option. This is a major security flaw for any use case involving verification codes, password resets or account credentials. Automated bots actively monitor popular YOPmail addresses.
Private inboxes stay locked. Only your browser session has access. It's more secure for receiving sensitive information. Even if someone knows your Guerrilla Mail address they can't access the inbox without your specific browser session.
Email Retention
Emails stay for 8 days. That's long for a free disposable service. This window helps if you need an email days later. You just have to remember the address and accept that others can see it too.
Your inbox expires after about 1 hour of inactivity. This is a short retention window. You need to use it right away and copy any important information before the inbox disappears. This timeframe is tight for anything beyond quick verification.
Sending
Receive-only. Cannot send or reply to emails. If a service requires you to respond to an email as part of the verification process, YOPmail cannot handle it.
You can send outgoing emails with this service. This is a rare feature that sets Guerrilla Mail apart from most disposable services. Deliverability varies. Some recipients accept mail from Guerrilla Mail servers. Others filter it as spam. The capability itself is unique.
Interface
The site is packed with ads. Pop-ups and overlays get in the way of your inbox constantly. On mobile devices the ads make the service almost impossible to use. Advertising is how they make money and the poor user experience shows it.
The interface is dated but functional with moderate advertising. It is less intrusive than YOPmail. The design hasn't been updated in years but the core functionality remains. You can read emails and check inboxes easily. It works without ads getting in the way of your tasks.
Domain Blocking
yopmail.com and variants are on most blocklists after 20+ years of operation. No alternative domains are available. If a website blocks YOPmail, you've no recourse within the service.
Guerrillamail.com and its variants are blocked just as often. Both services face the same core problem of long-established domains that every blocklist targets. Guerrilla Mail offers a few domain variants like grr.la and guerrillamailblock.com but these are on blocklists too.
Custom Address
You can pick any name you want for your inbox. Because the system is public, anyone can guess your inbox name and access your messages. Choosing a predictable name like your own name creates a security risk because anyone can navigate to it.
You can pick your own inbox name. This setup works better and is more secure because you create a memorable and natural-looking address while knowing that only your session can access the resulting inbox.
Verdict
Guerrilla Mail is the better choice for most people. Private inboxes are a major advantage over the public model used by YOPmail. This is important when you are receiving verification codes or password reset links that could be intercepted by anyone.
YOPmail's only real advantage is that it keeps emails for 8 days instead of 1 hour. If you need to check your messages days later and privacy isn't your main concern, YOPmail has the edge. But because the inbox is public, that retention window is shared with anyone who visits the same address.
Domain blocking hurts both services. NukeMail offers private inboxes like Guerrilla Mail. It provides better retention than either with 24 hours active plus 14 days locked. It also keeps fresh domains that aren't on blocklists.
If you're picking between these two legacy services, the choice depends on whether you prioritize inbox privacy or email retention. Privacy should win in almost every scenario. A verification code that someone else can read is worse than a verification code you have to grab quickly.
Neither service has evolved in recent years. Both face the same challenge of blocked domains that limits their usefulness on modern websites. Newer services like NukeMail combine the best aspects of both. Private inboxes like Guerrilla Mail offer longer retention than either. They add features like custom address names and multiple fresh domains and access codes for returning to your inbox from any device.
If you use YOPmail for its longer storage, NukeMail offers 24 hours of active access plus 14 days of locked data. That is 15 days of data preservation. It is nearly double the 8-day window you get with YOPmail. Data stays private the whole time. In the locked state you can see sender names and subjects. You can unlock full content with a premium upgrade if you find you need it later.