Merriam-Webster's indicates that they are synonyms for each other and does not provide any meaningful distinction between them in terms of the example sentences that you give. So, I can't give an objective answer.
However, I do believe there is a difference in usage and meaning.
To explain, I will first use use a different scenario along with several different sentence constructions.
Bob started a new job last month that requires he wake up at 7 a.m every morning. This is a change for him, as, for years now, he would wake up at 8 a.m.
One morning, Bob leaves his house at 8:30 a.m.
Bob woke up recently. [He woke up just a short time ago.]
Bob woke up lately. [This is ungrammatical.]
Lately, Bob has been waking up at 8:30 a.m. [His schedule has changed from what it used to be a short while ago.]
Recently, Bob has been waking up at 8:30 a.m. [This is technically correct, but lately would be the preferred word as recently sounds a bit odd here.]
In considering these examples, I would say that recently implies a single event that has occurred a short time ago. Lately, however, implies a change in behaviour or circumstances, as well as referring to something that is more prolonged or recurring.
Returning to your actual examples:
I've been very busy recently. [Your basement flooded last night. It was an unexpected event that you've been dealing with to the exclusion of everything else. You think you have it fixed now.]
I've been very busy lately. [You weren't so busy before your child was born. Now, you always seem to have much less time. Your lifestyle has changed.]
Addendum: Following an offline chat resulting from comments, I thought it might be useful to list the verb tenses I believe are compatible with the two words.
Simple Past—"I ran." Recently only, although perhaps lately when used poetically or archaically.)
Present Perfect Passive—"I have been run off (the road when . . .)." Despite my initial misconception, this is not in the Present Perfect Progressive. Both work best with an extended event, like "I have been sick."
Past Progressive—"I was running (when . . .)." Recently only.
Present Perfect Progressive—"I have been running." (Lately would be preferred to recently, although recently is technically correct.)
Best Answer
The simplest way to think about these two terms is perhaps,
Sometimes the two terms are somewhat overlapped.
From Wikipedia,
From Microsoft,
(this is specific to Windows operating system, but the concept is similar to user account and user profile in web programming)
(emphasis added)