There is no difference in cost or service time between calling the guy up mid-July and calling him up in September.
If they wait until 2 weeks in April 2019, that would save a year of service time and buy us an extra year of free agency. I think Eloy Jiminez may accidentally trigger nuclear fusion hitting a baseball too hard at AAA if we make him wait until April 15.
If we call either of them up now, they would be "Super 2 players". It would NOT cost us a year of control, we would have both of them 2018, 2019, 2020, 2021, 2022, 2023, and 2024. However, if they are called up now, or any time before ~mid July, the called up players would be arbitration eligible in 2021.
If they're as good as they look, if they are called up now, in 2021 they would each be paid ~$12 million, and their salaries in each subsequent season will be higher. On the other hand, if they are called up after the Super-2 deadline, called up in mid-July, then in 2021 they will be minimum salary players again and their first arbitration year will be 2022. Effectively, each player will probably make another $20 million in total prior to 2024 if they are called up now compared to what they will make if you wait until mid-July.
If you call them up now, they have 4 arbitration years. If you wait until mid-July, they have 3 arbitration years. If you wait until April 15, you add in an extra year of control through 2025, but that year is an arbitration year and the player would be back to having 4 arbitration years.