Hi Plgorman.
Welcome to our Mattress Forum! 
I am sorry to hear about your medical condition and I understand that you are trying to make the best out of this situation within a limited budget. One thing to keep in mind is that because of your preexisting condition the cause(s) of your back and neck pains may be compounded and you will need to investigate if the pains you are experiencing are due to the herniated disk or due to support/comfort issues or a combination of both. You need to do more of an extended local or in-home testing to find the best sleep system for you and the construction that best matches your “sweet spot”. Nest has the advantage of a good 100 nights of return/exchange policy which should give you enough time with any of the trial and error that may be needed.
As far as choosing between the two mattresses, it is impossible for me to diagnose someone with a very specific medical condition and determine what may or may not work for you via an online forum… while I can certainly help with “how” to choose … I cannot make specific suggestions or recommendations for either of the mattresses or the combinations of materials or components that would be suitable for you because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
I understand that because your limited budget you may not wish to go with the recommendation that Nest had for a Q3 All Latex Mattress but if you want the benefit of latex you can certainly could go with their hybrid line up mattress using spring and latex on top which can work just as well and meet all your needs. In fact many people prefer the feel of latex over sprigs, but again I wouldn’t know what you would respond best to and you would need to carefully select the right level of comfort/support for you because of your herniated disc condition.
As you may be aware Nest Bedding is using high quality and durable materials and they are one of our site members which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of quality or their products, value, service, knowledge, and transparency. I would certainly suggest to you to rely on Nest’s advice as they are much more familiar with their own mattress designs and materials than anyone else (including me) and they can use the information you provide them about your body type and sleeping positions, your preferences, your history on different mattresses, and the results of your local testing to make suggestions based on the “averages” of other customers that may be similar to you.
I would be interested to find out what you decide and how things progress for you.
Phoenix