I think that it's better to do an elective in a subspecialty then to do a sub-I at another institution. Sub-I evaluations do carry more weight and attendings do usually get to know you better in them, but electives have several advantages over sub-i's. Your performance in a sub-I may be hindered by your unfamiliarity with the way things are run at the hospital, this will be less of an issue during an elective where you will have fewer responsibilities. I've noticed that some attendings have lower expectations on student performances during electives, so if you take your work seriously (ie offering to stay late or come in early), it will be easier to impress an attending during an elective then a sub-I. Finally, the fellows in certain specialties who do not get that many students doing their elective with them are oftentimes very appreciative of having a student on the service with them who will see patients for them while they are doing other work. This can be very valuable in ensuring a strong letter of reccomendation or evaluation too, since attendings frequently rely on fellow and resident feedback for what to write in their evaluations.
October is technically not too late to ask for a LOR since you can add additional letters of support to your application to some programs after submitting your application in ERAS (earliest submission date is ~Sept 1st), but it would be preferable if you could do your elective earlier in the year so that the letter can count as one of the required letters to complete your application. Remember that some attendings require some amount of time to write their letters of reccomendation too. Some programs that you will be applying to will also not allow you to send additional letters beyond the required 3-4 letters (most programs require 3 letters including your chairperson's letter, some will allow you to send 4-5). Some students do not just use their away electives to get letters to send to different program though, doing an away rotation during October, November, or December can still be advantageous in showing a strong interest in the program and you can sometimes meet with the program director or chairperson to discuss your interest in the program during your rotation. If the attending really likes you, he or she may be willing to make a phone call for you too or put in a good work with the PD. Just be cautious about taking off too much time during your away elective if you do one in December or January, as even though most attendings expect that you will need to be away for interview purposes, it probably doesn't help you look like a dedicated student. On the other hand, another consideration is that if your away rotation is in a city where a number of programs that you are interested in are located at, you could save money on traveling cost by doing your rotation during an interview month. I don't think that there's anything wrong with taking 3-4 days off for interviews during an away elective for interviews, I just wouldn't try to schedule too many interviews during an away elective. I would tell you what Iserson's reccomends, but I can't find my book right now.