which swim workout is better for increasing speed?

Probably not what you want to hear but....at 2:00/100 yards, you'd probably see the most gains from improvement in your form. Not sure what drills you're doing or what kind of feedback from a coach you're basing them on, but my guess is there are still major issues with your form that are holding you back, and the drills you're doing aren't solving them, nor are they really helping your conditioning. Is there any chance to have someone take a look at your swim and make suggestions?

As for the workouts, both are useful in improving conditioning. The shorter repeats for speed, and the longer ones to build endurance and comfort at swimming at different paces/effort levels. The would be typical of my two shorter weekday workouts. Especially for novice swimmers, I think the occasional long continuous swim is also helpful, to work on endurance and build confidence and a sense of pace.

I do wonder if fatigue from the short rest intervals might be causing you to reinforce inefficient habits. If conditioning is the goal, then a shorter rest interval is a good idea. However, if there are major flaws in your form, then taking a bit more rest might help you to focus more on maintaining an efficient stroke throughout, which will ultimately make you faster. One workout I find really useful for the latter goal is a set of 100's (on 2:00 for me, which gives me about 25 seconds of rest) where the first 25 is easy (focusing on form), the second build (starting easy and ending hard), the third another easy (focusing on form again) and the last hard. I choose something specific to focus on, either for the entire workout or a each portion of it, especially during the "easy" part, then try to "hold" that during the other laps. I've definitely noticed an improvement in my ability to maintain a smooth, powerful stroke even when quite fatigued after doing many of these and similar workouts.

Either is fine but I would drop the 500 out of the pyramid.  You could add something like this to make it the same distance.

5 x 50 descend 1-5 on a pace time that gives you ~5-10s rest
100, rest 10s
200, rest 20s
300, rest 30s
400, rest 40s
500, rest 50s
400, rest 40s, faster
300, rest 30s, faster
200, rest 20s, almost sprint
100 sprint, rest 30-45s
5 x 50 descend 1-5 and try to hit the same times as the first time through 

I should note that last year was my first year swimming for sport ever.  Up until a couple of months ago I had never done a single drill.  I am working with a coach and trying to get some structure around my swimming.

Have you asked the coach to help you plan out some effective training?  I wouldn't expect them to write out workouts for you if you're just paying them for some stroke work now and then, but they should at least be able to help you with the structure of what to work on.

Working with a coach? Or an instructor? A coach should be already giving you structured workouts with specific purpose. An instructor teaches you how to swim effectively. They are not necessarily one and the same person (And often aren't).

John

If your typical workout is 2-2500yd then a main set of 2500yd is too long for speed work. Also, you get way too much rest. For speed work do short(er) intervals, short intervals will have relatively more rest time so don't expect to do the same total as a normal workout.

If you can't do repeat sprints then you can do progressive, say 1-3 or 1-4 the first being normal, the 3rd or 4th being max - no easy swimming, and no extra rest after max. 

Examples of speed sets:

12x25 max leave at :40 - you should have 10-15 sec rest

12x50 progressive 1-4 leave at 1:10 - you should have 5-15 sec rest

4 x (

100 normal 80-85%

75 hard 85-90%

50 very hard 90-95%

25 max 100%

)  10 sec rest between each interval

If you do a short sprint set such as the first, you can follow with a hard set, that is, longer intervals but slower. This is still speed focused, but speed maintained. For example, combine:

12x25 sprint

12x50 progressive

12x100 hard

You should run out of juice by the end of the hard set. Or you can do another set with focus on either technique or strength/endurance.

"(I seem to be stuck right at 2:00/100y over this long distance)"

If it makes you feel better, I just swam a 1250 and averaged 2:00/100 and I was overjoyed!  I'm a pretty new swimmer and just recently have been swimming with a Masters group.  Masters makes a HUGE difference as you all know swimming is so technical.  I know I will never be able to catch up to the group of high school and college swimmers so my  plan is hold my own on the swim and catch them on the run!

Good luck!