The first difference I would like to examine is not the Sabbath, but rather works and their role in assurance.
It should probably be noted that the different places these works of a believer are discussed is probably important. The Heidelberg starts with our sin, moves to salvation in Christ (where works are denied), then hits the sacraments before going on to Christian works of thankfulness. The WCF starts with creation, sin, salvation in Christ (where works are denied), and then goes into works, assurance, the Law, and even civil government before getting around to the sacraments.
Let us start with WCF 16.2: "These good works, done in obedience to God's commandments are the fruit and evidence of a true and lively faith and by them believers manifest their thankfulness, strengthen their faith . . ."
The WCF starts by letting us know that good works are an evidence of our salvation in Christ. Thus, we can expect to see works as a major portion of our assurance of salvation. The article does mention the role of works in strengthening our faith among other things. The section does go on to mention the imperfection of even these works, but still the works are a reliable evidence of our salvation. WCF 18.1 then does follow this logic by stressing assurance can be had for those who "truly believe in the Lord Jesus, and love Him in sincerity, endeavoring to walk in all good conscience before Him". Notice the assurance seems to be looking primarily at ourselves including our works, which I would say falls under walking in a good conscience before God. 18.2 gives the ground of assurance as the "divine truth of the promises of salvation, the inward evidence of those graces unto which these promise are made, the testimony of the Spirit witnessing with our spirits". While this does add the outward promise of God the other are inward focused. The Westminster Larger Catechism Q.80 answers assurance by combining those two making the stress again fall upon those who are endeavoring to walk in good conscience. Absent from the assurance section is any mention of the sacraments.
The Heidelberg takes a very different view focusing outward on Christ for assurance. Assurance shows up in Q65 about the sacraments where the Holy Ghost confirms our faith by the use of the holy sacraments. Now the WCF uses the confirming one's faith language in their section on the sacraments, but the sacraments themselves do not show up in the section on assurance and works do, while works themselves have already been discussed. The Heidelberg begins with the sacraments, which "declare and seal to us the promise of the Gospel". Thus, assurance is going to come from the means of grace, which are designed to direct us not to ourselves, but to the "sacrifice of Christ on the cross as the only ground of salvation" (Q.67). Then the in-depth discussion of both sacraments is slamming that fact home. How do they direct us to the cross? Where can you find that in the word? Is the power in the sacraments themselves? Now in HC.Q86 we do see the Heidelberg say that works have the function of assuring our faith by the fruits thereof, but it still comes out as third in a list of four. Let us not forget that the Heidelberg has already taught us that our good works in this life are all defiled with sin (Q.62). The discussion of the commandments is next, and it ends with this reminder that "even the holiest of men, while in this life, have only a small beginning of such obedience" (Q.114), and that the law for the believer is still ultimately to teach us our sinfulness and drive us to Christ for forgiveness and to Him in prayer for sanctification (Q.115). It is not quite the same view of the Law shown in WCF 19.6 where the law itself is useful to the regenerate to "restrain their corruptions". The Heidelberg seems to focus on the law as showing our corruption and leaves the making us ready and willing to live unto God up to the Spirit (HC#1).
So it appears a very different method of assurance is found in the Three Forms as opposed to the WCF. The WCF points heavily to oneself and the personal witness of the spirit and their own works, to serve as evidence of salvation and thus assurance. The Heidelberg points much more toward preaching and the sacraments as the main route of assurance since those both point toward Christ on the cross. The Belgic seems to agree more with the Heidelberg as it states in Article 24 if we looked to works "we would always be in doubt, tossed to and fro without any certainty, and our poor consciences would be continually vexed". No assurance that does not point to Jesus Christ. Although it has to be admitted that the Canons of Dort 5th Head of Doctrine seem to point inward to a witnessing of the Spirit to one's spirit, and to one's work as the way to assurance.
In summary assurance for the WCF is to look within oneself to see the fruits and for the Heidelberg and Belgic it is to look to what Christ has done.
I open the floor for discussion and rebuttal.