to start with: why into a single report/document?
apart from that, it is possible with both, so the "best" method is "whatever you like"...

SSRS is preferred.

The reason is that all of the documents are parts of a whole contract. I want to make it as easy as possible for our staff who hands out these contracts and to be able to have it generated and exported as a single doc would be helpful.

There is a report scheduler, which can join multiple reports in one document. You can add multiple Crystal reports, SSRS reports and pdf files,  produce a single document and send it by e-mail. The tool provides also a workflow, which will allow you to add a report or pdf file if a condition is satisfied. For example you can add a PDF "Welcome letter" just for the new clients or a report with local offers just for clients in a specific ZIP etc. The tool provides also a WYSIWYG editor for the e-mails, which supports mail-merge .
Check their website for more details: http://www.r-tag.com/Pages/ReportManager.aspx

Disclaimer: I was working for this company in the past.

There is at least one other 3rd-party Crystal Reports desktop scheduler among those listed at http://kenhamady.com/bookmarks.html that can automate the process of merging multiple Crystal reports into a single document.  The merging can be done not only for pdf files but also for MS Word files (which could be relevant for your case where you need to handle contracts).  PDF merging can also take care of generating bookmarls, page numbers, and Table of Contents.

In addition to the previous answers: one limitation of SSRS you may want to take into account is that it's not possible to mix portrait and landscape in PDF exports.  Imagine you'd had a requirement to have the first page, which is a letter, in portrait while all following pages should be in landscape.  That wouldn't be possible without additional tools or custom code.

Besides that, I don't see any issue if you'd go in the "main report with subreports" direction.

ValentinoV is right - with SSRS you can use subreports and the only problem will be if you need mixed (portrait AND landscape) orientation of the pages. With Crystal report you will have one more problem: this engine doesn't support nested subreports hence your individual reports cannot contains subreports otherwise you will be not able to add them as subreports to your main report. It might sound confusing, but here is an example:
You have 5 reports which you want to export to one file. You can add them as subreports to a new report, which will be used just as a "packager". When you export the "packager" the exported file will contain the result from all 5 reports. If one or more of these 5 reports contain subreports then you will be not able to add it to the "packager" because you will have nested subreports and this is not supported by Crystal. SSRS will be able to handle this scenario without problems.

Using an external tool will allow you to have portrait and landscape pages (in both SSRS and Crystal) and to use reports with subreports in Crystal. I like R-Tag because:  
 * It is written using the latest technology and has 32 and 64 bit versions. No old COM components covered with a new skin
 * Supports both Crystal Reports and SSRS
 * Can control the order and existence of the reports and the static files based on custom conditions
 * Has internal document management, C# compiler and tons of options to customize your export including a WYSIWYG e-mail editor and export to SharePoint
 * It might be helpful in other projects: for example you will be able to use data driven subscriptions even with SSRS Express

However, keep in mind that adding one more item to the chain will require additional time for training and implementation, so ANY 3rd party tool will come with a hidden price.

This appears to be quite cumbersome. I have multiple pages that I need per subreport/doc. How do I create "pages" in SSRS? I thought about trying to group but I am not sure what to group on, I know I can create page breaks that way.

SSRS creates pages automatically when you export to a format that supports pages (such as PDF).  It will use the PageSize and Margins report properties while doing so.

Have a look at the following webpage to get a better understanding on how pagination works: Pagination in Reporting Services (Report Builder and SSRS)